Repost from Ubuntu Blog.
You can install alien itself from the Ubuntu Universe repository by adding the repository to your list of sources and doing:
$sudo apt-get update
$sudo apt-get install alien
Installing the .rpm file
To install the .rpm file, you first need to convert it to a .deb file which can be installed on Ubuntu.
I assume that you downloaded the package to your Desktop (~/Desktop is the directory)
You can convert the .rpm to a .deb by using the following commands.
-This will change the directory to your desktop, where you have the .rpm file.
$sudo alien -k name-of-rpm-file.rpm
– This will convert the .rpm to a .deb.
– The “-k” will keep the version number. Otherwise alien adds a “1″ to the version number.
– Tip: Use Smart Tab Completion to avoid mistyping the file names
$sudo dpkg -i name-of-deb-file.deb
– This will install the .deb package
Try reading the alien manpage for more details on how to convert other kinds of packages and the options available.