Hi, welcome to the Magix forums.
First, I have to say I don't own Acid so am not able to directly answer your question, but not all mp3s are created "equal". There are so many possible "variations" in the creation of them: bit rates, sampling frequencies, CBR, VBR, stereo, mono, etc., etc.
So, is it every mp3 of whatever type that fails to be recognised?
One other thought: I assume you are running a full, purchased, version of Acid Pro 8, not just the trial? If it is the full version have you downloaded all the additional content for the program? Perhaps that includes an add-on needed for mp3s?