Hi, the mouse buffer is automatic. You select some text with the mouse, and *bam*, it is in the mouse buffer! To paste it from the mouse buffer you have to middle-click your mouse (click the scrollwheel if you do not have 3 buttons). If you don't have three buttons, or a scrollwheel doesn't click, press left+right button at the same time.
Linux Mint 14 Nadia / 64-bit / Cinnamon