Info-Mac

For a whole my mac has been really slow. Even when typing, the letters took seconds to appear. Then, last night, I couldn't get on the Internet. I have a Time Capsul. It was showing the green light but no Internet. I restarted it. Then I got a message- something like: "this apple wireless devise doesn't have a DSN address. It may have trouble connecting to the Internet." I'm a bit clueless about this stuff. How do I find out what my DSN address is? And how did I lose it to begin with?!

If by DSN you mean DNS, then for some reason your router isn't getting a DNS server from your ISP. You might try manually plugging in some generic DNS like 4.2.2.1 and 4.2.2.2 and see if that helps you there, like so:
If you want a bit more flexibility you can try opening a free OpenDNS account and configure your router and/or Mac to use that.

Most likely your WiFi access point and/or modem got interrupted and needs to be rebooted so the embedded DHCP server can assign a local DNS cache. You may also need to set it up again with the log in info for your ISP account, as well as any firewall or security settings. Most WiFi routers let one save a configuration file to make this easier to redo. The typical IPv4 address for the admin account is <http://192.168.2.1/>. There is also a DNS cache that Mac OS maintains. This can be cleared by running one of the following scripts from the CLI (Terminal.app):
for 10.5 -10.6 (Leopard and Snow Leopard): dscacheutil -flushcache
for 10.4 (Tiger): lookupd -flushcache