Hi there! My guild uses Guild Connect. There are two reasons for the "Login Rejected" message - the simplest is that, indeed, you've been given the wrong credentials. The slightly more complex reason is down to where you downloaded GC from. If you didn't download your guild's specific downloadable, you'll probably have the wrong server/guild details! All is not lost though...
Look at the folder where you have GC installed. You should have a file called "Instance.txt". That file (open it in a text editor - like notepad) contains a server port number and a string that represents the EQ2 server you play on, with your guild name. For example:
If the information there looks wrong, ask your guild leader/GC admin what they should be (you can probably guess the servername/guildname bit but the port number wont be guessable). Correcting that will hopefully enable you to login