Working from memory here, my acoustica is on my home pc which I havent got next to near in the last 3 weeks due to work commitments.
You have to insert a sound file(your mp3 file) into acoustica. This will then show up graphically as a really long wave format. I cant remember where this is on the menu bar but I think its in > file > insert. In your case as you just want to cut a bit off the end, scroll to where the sound waves taper off to indicate there are no waves. Select the bit you dont want and delete. You then need to save your edited wave file as mp3 so thats definitely in >file > save as ...
I mostly use acoustica to split recordings. I record live sessions to mp3, import to acoustica, find the splits in the songs by finding where the sound waves are low or non existent, set a flag marker, edit the marker and say call it song 1, set your next flag at the end of song 2 and rename it so. Then click file> save as mp3> and it will use the flag markers as the mp3 file names.
Tip, Decrease the zoom on the wave file if working with bigger files so you can get the entire wave onto one screen. Set what you think is the marker and then zoom in again and fine tune the set.
Thats pretty much it, play around with it as theres not too much to it as it really is a very basic wav/mp3 editor (hope I dont sound condascending).
If you try any of the above and get stuck mail back and I'll try to help more.
(If you think thats confusing you should try Ableton live. sheeeshhh...)
