Adding Audio Clips
There are several ways to add audio clips to the project.

Click Add Sound.. From The Mix menu.
Navigate to a folder with the sound, select it and click Open.  The sound will be placed where the caret was.  Alternatively, you can click the Add Sound... option on the Sound menu (Ctrl+H) or click the Add Sound. button on the toolbar.

Double Click The Sound Workspace Area
When you double click the sound workspace, you set the caret and simultaneously bring up a window letting you choose a sound.  The sound will be placed where the caret was.

Add A Sound From The Loop Library
Click the Library Tab on the bottom of the Mixcraft window or click the Show Loop Library button on the toolbar. Select a sound from the library, position the caret where you'd like the sound to go and then click the + button next to the sound.  Alternatively, you can drag the sound from the library directly into the sound workspace.

Drag A Sound In From Window's File Explorer
Open a Window's File Explorer window and navigate to the sound or sounds that you want to add.  Select the sound(s) in the File Explorer and drag them into the sound workspace.

Sound Analysis During Loading
When a sound is loaded, the software parses it for peaks in order to display the wave form.  If Auto Beat Matching is on, the software also parses for tempo, beats and key (for sounds over 30 seconds).  If the software detects a tempo or key, it may adjust the snap point to the first beat and alter the sound based on the project key and tempo.

For more on adding sounds see How To Manipulate Sounds.