Mix in wav or Mp3: find out the two locators in that black line with time or frame square under you icons bar. They are in left corner. (small triangles). Than grab one and pull it to the end of your mix and pull another to the start. Than go to File -> Exiport and select your mixdown export file tipe and name. If is not working just swap those two locators.
Mp3 inport: Add new audio track, than select that track and go to File -> Import -> Audio file... Ther find your file and import it.
For left right control (pan control) click on your audio track, than cilik on small + in the left corner. Click on the 'volume' tab and select pan left right. Than click one line lower and you will be able to control pan.
There could be a problem if you record your track stero, but you used only one channel. You'll have to convert that tracks to mono before or record both channels.
Sorry for my English.