A known bug on Pool Linux is that the mouse cursor is sometimes invisible on the login screen. Fortunately, there's enough keyboard control to get by.
To begin with, you'll see a rectangle with the University of Cambridge crest, the machine name, and a button labelled “Log In”. Pressing Return will get you to the next stage, where your username is requested. As usual, enter your username, then Return, then your password and Return to log in.
After you enter your username, you can choose your language, keyboard layout and desktop environment (“Session”) — or at least, you could do easily if you could see the mouse cursor. However, there is a workaround that is unfortunately rather unintuitive.
By pressing Alt-L, Alt-K or Alt-S, you can change the language, keyboard and session respectively: all three keys open the choices available and can be navigated to with the cursor keys and Return. The tricky bit is getting back to the password field to type it in and log in, as there is no accelerator key for the password field. The quickest way is to press Alt-C to cancel the login dialog and log in again. Your changes to the session, etc., will still be there when you re-enter your username.
Shutting down or restarting
The power button should be easy enough to access even without a visible cursor. Just move your mouse to the lower right and the power button should be highlighted. Click on it and you can then choose what to do with the cursor keys and Return.
If you really want a keyboard-only method, you'll need to get to the password prompt by entering a username (the username could be a dummy username, even consisting of one letter). When you're there, press Alt-S, Return, Tab five times, and finally Return.
This bug is annoying, but the workarounds do exist and are even relatively quick if you don't need to adjust any settings. I'll keep an eye out for any similar bugs that get fixed and will apply anything that works, but given the severity of the bug I may be chasing more important ones first.