Are you saying that you have the video imported into the software, but have no audio, and now need to import the audio ?
If so, that should be very straightforward. First, choose the "timeline" view of your video. You'll see separate tracks for the video and the audio.
Now, import your audio wav/mp3 into the current open folder (which generally covers the central part of the screen in the versions I've seen). Once it's in there, just drag and drop onto the audio channel in the timeline. Voila !!