Hi Eric,
What you are probably having trouble with is the 'URL Patterns'...
Open one of your accounts, and click the URLs tab...
If the list in the box is empty, that is your problem. If it isn't, then there is probably something incorrect in the pattern you entered.
For Yahoo accounts, it should look something like this:
https://login.yahoo.com/*
The trailing asterisk is important... without it, the pattern won't work unless you specify the entire string - and that is unreliable, because that long string can change.
Do this for each of your Yahoo accounts, and then you will get the Account Selection box, to let you choose which account you want to log into Yahoo with.
I highly recommend that you
read the wiki about this, and while you're there, anything else you may not understand completely.
Feel free of course to post here with any other questions you might have...
Hope this helps...