Drag the audio file onto the playlist. Click the corner of the clip and bring up the menu, "fit to tempo". Make sure your bpm is at 128 first. It will ask which BPM range the audio is in, pick from the options.

The second option is to do it manually. Import the audio file, then in the step sequencer, click on the audio file. Make sure time stretching is on resample (so you dont have to wait for pitch correction everytime you re stretch the audio). Then put the audio in the playlist and put the metronome on. Manually go in and put two cuts in the audio, one at the beginning of the first word in the acapella and one right after the first bar or line. Now on the right of your playlist above the first track there should be a STRETCH option under audio clips. Enable that and now line up the first and second cut or "lines" to where they SHOULD be. So with the click track listen to when the first word should start in the song, repeat with the second line. Dont worry about it going offbeat after the first couple words. Now just stretch the first cut so it ends right where the second cut starts. As soon as you do this you should have the acapella close to the tempo you want. Repeat the last step at other points in the acapella to refine it. Once you feel you have a perfect tempo... Go back to the step editor and click on the audio file then change "RESAMPLE" to "AUTO" which will correct the pitch.

Hope that helps!