First try "import Tkinter" with a capital "T". If that doesn't work, post the code you used and an exact copy of the error message.
If you have to install, install python-pmw. It will install TCL, Tk, and Tkinter as dependencies if they aren't installed, and you will likely want the PMW extensions if you are going to program GUIs. If it still doesn't work, then re-install python via Synaptic or apt-get on the command line. On some distros, the installer will check for external packages and make python accordingly. That is, if there is no Tkinter installed, it will not configure python with Tkinter, so you have to re-install python so it can be re-configured.
Finally I solved the puzzle of installing Tk onto Ubuntu. I tried everything I could think of; download from CPAN, download from Perl/tk
Finally I used apt-get TK. These were the magic words one must use.
Perhaps you should try this. I don't have ubuntu, so I try to help you from very far away :)