Welcome to BZFlag.
I'm sure you've read the Getting Started
When starting the 'BZFlag' program, you should see a menu with the options listed below:
Use the 'Enter' key on your keyboard to select this option.
You should now see another menu with the options listed below:
The username you have in forums will be the name you'll enter in the 'Callsign' field.
The password you have created on forums will be the same password you enter in the 'Password' field.
(You'll have to use the arrow keys to move to the correct field.)
Move to the 'Team' field now and use the arrow keys to select automatic, observer or a team which is available.
Move up to the 'Find Server' area and press the 'Enter' key.
You should now see in your window something which looks like a list.
Use the arrow keys to select a server you wish to join.
Press the 'Enter' key and it should bring you back to the previous menu.
Now move to the 'Connect' field and you should be able to join the server.
If you're unable to play for whatever reason, it might be due to the reasons listed below.
1. You're joined as observer. (Observers can watch, but cannot play while in this mode.)
2. You've joined the wrong team. (The server will generally kick you if you join the wrong team.)
3. You're blocked from the server. (This is rare, but sometimes abuse might come from the same host and is probably meant to block someone else.)
4. The server is offline. (Yes, it happens sometimes, but it is also very rare.)
5. The server is set-up to only permit certain people to use it. (Some server owners will set-up the servers to only allow people from a certain global group to use it.)
By the way, if while you're searching for a server, and you notice something in red which says something like "notification" or "notoken",
it is a notice which is given.
If it's a notification, it means someone has sent you a private message through the forums.
If it's a "notoken" or similar, it means you're callsign or password might be incorrect or could be when authentication is being maintained.
Try re-entering your password and callsign and be sure to see that they match the ones used in forum.
If this does not solve it, try to join a bit later and it should work.