Use top or type ps -aux to see if its running. Unless its running in part of inetd or xinetd, etc. Check your xinetd.conf file or directory in /etc and see if you have a configuration setting there for swat to be running, etc. Post output here.
I couldn't tell what the problem is if that is the IP your using and swat is running. Can you put the error message here you get when trying to access it, as if its denied or 403, etc? Try from the same machine like: http://localhost:90
and see if that works. You can't type in your router's address, as it wouldn't be running from on it. If your trying to access outside your router, put in your actual IP address and forward all port 90 requests to your IP of your machine its running on, etc.