Use Alien:
alien -d /path/to/.rpm
creates a deb package from the rpm package.
Basically you should try and download .deb (debian) files because they'll generally install better (as Ubuntu's based on Debian).
What the alien command is essentially doing is converting the file (*.rpm) to a Ubuntu friendly format (*.deb).
How can I install the .deb file?
$> sudo dpkg -i yourfile.deb
or
$> sudo dpkg -i yourfile.deb && apt-get -f install
To get rid of dependency problems.
