It's either taped to the bottom of the router or :-)
There are 4 ways to implement wireless security in a Linksys router, Disable SSID (broadcast of wireless name), WEP (Wireless Equivalent Privacy), WPA (Wireless Protected Access), and MAC address filtering. These are probably listed in order of ease of use and degree of security and are covered here: http://linksys.custhelp.com/cgi-bin/linksys.cfg/php/enduser/std_adp.php?p_faqid=4024&p_sid=UcVbWIYi&p_lva=3967
Not broadcasting the SSID doesn't make it invisible, it just doesn't send out the name. I guess an enterprising 12-year old could probably figure it out, but it does keep the casual user out of the network. MAC address filtering is the most severe, but it means that every user's MAC address has to be discovered and allowed in your network. Not much fun if you have friends over or test a lot of wireless equipnement.
In a Linksys, network keys come in two flavors, WEP or WPA. The link above explains the difference. The easiest one for most of us to use is WEP. It's not as secure as the WPA, but it keeps the casual user out of our networks. I imagine the "hacker" can break the code, but I'm just concerned about the neighbors. Both schemes will slow down the wireless...like you're really going to notice :-). I still do my laptop drive synchronization to the file server via wireless and it runs overnight.
To implement 64-bit WEP, you need a 10 character HEX key. I pick something simple like 1011121314 or FACE123456. (hex being 0-9, A-F). In our Netgear routers, It stays visible in the router so if you forget, you can see it. I'm pretty sure that the longer the key, the more overhead is added to your transmission, but you can make your WEP key 128 bit if you think 26 characters is better than 10. If you use WPA, you can pick your key like (Wir3le55), but if you don't spell it right, or forget it, you have to reset the keys.
This visibility of a WPA key may not apply to Linksys, but it's something to check.
You can easily spend an hour or more on the Linksys link trying to understand what to do with security. When I did mine, I didn't find out until I was on vacation that I even needed it. We get activity reports of websites visited emailed to us, and the reports kept coming in! It turned out that one of our neighbors was using our internet while we were gone. I never found out which one, but we now have 4 or 5 networks to choose from LOL.
So, the quickest way back into the router is to turn on WEP, give it an SSID name, give it a 10 character HEX key, and save it, then go to your laptop, log into the Wireless with the same key and you're online.
Fast explanation to a long post.