Here are the general steps you can take to add any audio file (including MP3) to your video clips.

1. On the File menu, click Open File.
2. Browse for the video and audio file that you want to combine.
3. Click Open to add them to the Collections window.
4. Drag the video file to the Video 1 track of the Timeline.
5. If the video file already contains audio then that will appear in the Audio 1 track of the timeline. Select this audio and delete it.
6. Then drag the audio file you want to combine from the Collections window to the Audio 1 track of the Timeline. Make sure that both the video and audio clips start at the same time.
7. On the File menu, select Make Movie and render the timeline to a video format of your choice.

To insert the clips at the current slider position, use the "Insert into Timeline" feature instead of "Add to Timeline" from the Edit menu.

Inserting into the Timeline will also move clips in all tracks by the duration of the clip being inserted. To avoid this, lock all other tracks by clicking the Lock option on the timeline.

After adding or inserting a clip into the timeline, you can move the clip to any position in the timeline by dragging it to that position. You can also cut/copy and paste the clip using the timeline.

For more about editing clips using Video Edit Magic's timeline, refer to the following article:
https://deskshare.com/Resources/articles/vem_EditingTechniques_removeunnecessaryfootage.aspx