if you go file - export as MP3 and get an error you need to go to the audacity site and downloa dthe LAME encoder and save it to your plug-ins folder usually under

C:\Program Files\Audacity\Plug-Ins
^ This.
My Gear:
Ashton AG131
Ashton GA10 (amp)
Marshall MG250DX (gasp, i know...)
Ibanez S470DXQM
EH Metal Muff
(")_(") ... he's gaining power
Alternatively, you can export as .wav and then convert with any free .mp3 converter. They're easy to find on google and such. I used to do this as I had problems with the LAME encoder.
There is poetry in despair.