ok...so ther eare two ways of configuring witter to talk to twitter. one is 'basic auth' where you provide a username and password.
The other is 'oauth' with this way witter will direct you to a page on twitter where Twitter asks to you authorize witter to talk to your account. You then take a code and give it back to witter at a prompt. From then on witter doesn't have to flow your username/password around to act on your account, it gets it's own key.
The idea there is that you shouldnt' really have to trust third party apps with your user credentials, just temporarily grant them trusted status which can be revoked at any time.

Anyway...in witter these are two very different means of talking to twitter. the baisc auth stuff is everything I originally wrote, and oauth uses a python-twitter library. I will merge these two over time, but for the moment it is relevant to any given error to know which path the code is going down.
I also encourage you to use oauth if you don't already, as that is the only way for twitter to know you are using witter in the 'source' field.

The fact that it shows a busy indicator when you hit the buttons is a good sign it is trying to do something, but if it's not giving any more results back, maybe I'm getting some kind of error under the covers.

so if you open up terminal and at the prompt type:
run-standalone.sh python2.5 /opt/witter/witter.py

that will run witter, but if you switch back to the terminal, you will see it output lots of information whilst witter runs, about various things it's doing.
So if you attempt to fetch more tweets, then go and look in that terminal, the last lines of output should be related to it trying to do that, and might have some helpful error message that would help me understand why it is not working.

oh and for future reference, rebooting or re-installing the same version, will never solve any problems with witter. it just doesn't work that way.