If you still need to install LimeWire, the instructions are very easy to follow:
Linux - Open a shell and cd (change directory) to the directory where you downloaded the installer. At the prompt type: sh ./LimeWireLinux.bin
What don't you understand? Also, you need to have a JRE (Java Runtime Environment) installed to use LimeWire.
As for other software, maybe have a look at this
. If the software you want to install isn't available through Synaptic, I guess you'd have to get the source and build it for your system (as fouldsy mentioned). I don't use Ubuntu, so I don't know the ins and outs of Synaptic..