This is how I added msilo to the kamailio default config. It still doesn't work for me (ie. offline messages are not delivered - but that's likely me messing up the routing), but at least Kamailio starts ok.

    $avp(oexten) = $rU;
    if (!lookup("location")) {
        $var(rc) = $rc;
        route(TOVOICEMAIL);
        t_newtran();
        switch ($var(rc)) {
            case -1:
            case -3:
                send_reply("404", "Not found");
                exit;
            case -2:
                send_reply("405", "Method Not Allowed");
                exit;
        }
    }

    # when routing via usrloc, log the missed calls also
    if (is_method("INVITE"))
    {
        setflag(FLT_ACCMISSED);
    }

    route(RELAY);
    exit;

            # we do not care about anything else but MESSAGEs
            if (!method=="MESSAGE")
            {
                if (!t_reply("404", "Not found"))
                {
                    sl_reply_error();
                };
                exit;
            };
            log("MESSAGE received -> storing using MSILO\n");
            # MSILO - storing as offline message
            if (m_store("$ru"))
            {
                log("MSILO: offline message stored\n");
                if (!t_reply("202", "Accepted"))
                {
                    sl_reply_error();
                };
            }else{
                log("MSILO: offline message NOT stored\n");
                if (!t_reply("503", "Service Unavailable"))
                {
                    sl_reply_error();
                };
            };
            exit;
        };

By the way, I get the 404 Not found reply (result of case -3) when the user isn't registered. Shouldn't an offline user return case -1 instead?

And here's the config that prevents kamailio from starting up.
I get the following error message:  (line 754, column 21 corresponds to t_on_failure("1"))

0(4603) : <core> [cfg.y:3411]: yyerror_at(): parse error in config file /etc/kamailio/kamailio.cfg, line 754, column 21: syntax error
 0(4603) : <core> [cfg.y:3411]: yyerror_at(): parse error in config file /etc/kamailio/kamailio.cfg, line 754, column 21:
ERROR: bad config file (2 errors)


    $avp(oexten) = $rU;
    if (!lookup("location")) {
        $var(rc) = $rc;
        route(TOVOICEMAIL);
        t_newtran();
        switch ($var(rc)) {
            case -1:
            case -3:
                send_reply("404", "Not found");
                exit;
            case -2:
                send_reply("405", "Method Not Allowed");
                exit;
        }
    }

    # when routing via usrloc, log the missed calls also
    if (is_method("INVITE"))
    {
        setflag(FLT_ACCMISSED);
    }

    route(RELAY);
    exit;

            # we do not care about anything else but MESSAGEs
            if (!method=="MESSAGE")
            {
                if (!t_reply("404", "Not found"))
                {
                    sl_reply_error();
                };
                exit;
            };
            log("MESSAGE received -> storing using MSILO\n");
            # MSILO - storing as offline message
            if (m_store("$ru"))
            {
                log("MSILO: offline message stored\n");
                if (!t_reply("202", "Accepted"))
                {
                    sl_reply_error();
                };
            }else{
                log("MSILO: offline message NOT stored\n");
                if (!t_reply("503", "Service Unavailable"))
                {
                    sl_reply_error();
                };
            };
            exit;
        };

 # if the downstream UA does not support MESSAGE requests
        # go to failure_route[1]
        t_on_failure("1");
        t_relay();
        exit;
    };

    # forward anything else
    t_relay();
}

failure_route[1] {
    # forwarding failed -- check if the request was a MESSAGE 
    if (!method=="MESSAGE")
    {
        exit;
    };
    
    log(1,"MSILO:the downstream UA doesn't support MESSAGEs\n");
    # we have changed the R-URI with the contact address, ignore it now
    if (m_store("$ou"))
    {
        log("MSILO: offline message stored\n");
        t_reply("202", "Accepted"); 
    }else{
        log("MSILO: offline message NOT stored\n");
        t_reply("503", "Service Unavailable");
    };
}