How do I enable the “multiverse” repository?
How do I reach the sites of Universe and Multiverse softwares for Ubuntu Free software?
Open Ubuntu Software Centre, then on the top system bar select Edit and then select Software Sources. Next tick the boxes next to universe & multiverse, enter your password and click okay.
I'm assuming you're asking how to enable repositories that are not enabled by default.
Open a terminal. Run
This will open sources.list in gedit as root so that you can make changes.
Any line in this file with a # is "commented", which means that it's interpreted as a comment and will not be loaded. If you see a part of the file you want to uncomment, you could change it from:
In the second option, the lines are uncommented and will be loaded by any programs reading from these sources (like the software center). Remember that you have to save the file after making these changes, then close gedit and the terminal and try finding your program again.