You're stuck there?.. If I'm right.
Continue with the installation. You could stay with the default options. And it should be fine and install. (Correct me if I'm wrong)
And be sure you have the Linux installer:
http://tremulous.net/files/Or here are some steps.
Step 1:Download the Linux installer:
http://tremulous.net/files/version 1.1.0
Step 1 1/2:Once it's downloaded
Right click on the file, and make sure it's executable:
http://ma65p.wordpress.com/2008/08/04/how-to-make-a-file-executable-in-ubuntu/Step 2:Open the terminal
Step 3:cd into the directory that you downloaded the Linux installer to. It might be your Desktop.
Try this in the terminal: cd ~/Desktop
Step 4:In the terminal type:
sudo ./tremulous-1.1.0-installer.x86.run
It will ask you for your password. Just enter it
Step 5:Go through the steps in the installer. And select OK. Accept License, and so on.
Step 6:Test the install of Tremulous, go to Applications/Games/Tremulous, or wherever else it installed it to.
If it works, continue the next steps.
Step 7:Go download the Linux installer for 1.2:
http://tremulous.net/files/It should be named: tremulous-gpp1-installer.x86.run
Repeat the steps 1-6, and rinse.

(Kidding about the rinse... just in case you don't know)
Of course for
Step 4, use the 1.2 file name, not the 1.1.0, it looks like this: tremulous-gpp1-installer.x86.run
Hope that helps