RPM files are for RedHat Package Manager, and generally not meant for Ubuntu...
actually the better/preferred way would be to use Repository using #apt-get install
since you have already downloaded RPM and wanna use the same, there are two ways you could follow
[] using 'alien' converting *.rpm RedHat package to a *.deb Debian Package like
*this you alrady performed
*This will convert your RPM to a DEB
{in your case should be flash-plugin-10.0.45.2-release.i386.deb}
and then you can install deb with:
#sudo dpkg -i flash-plugin-10.0.45.2-release.i386.deb
[] other way is converting RPM to CPIO, extracting contents of CPIO and installing from the source
#rpm2cpio flash-plugin-10.0.45.2-release.i386.rpm | cpio -dimv