Clicking in the margin of the score pauses/resumes the audio/video.
The menu is explained in the usage instructions
You can use the following keys:

spacebar	pause/continue	a	toggle autoscale
→	go to next measure	l	toggle line/measure cursor
←	go to previous measure	h	toggle help	f	toggle file buttons
+/-	increase/decrease speed	m	toggle menu	s	toggle speed/loop

In addition, when synchronization is enabled:

,	shorten the duration of the current measure	ctrl-,	shorten the initial offset (idle time before cursor starts moving).
.	lengthen the duration of the current measure	ctrl-.	lengthen the initial offset.
w or save button	save timings, abc code and other settings to a file (called a preload file). Also works with dropbox. You can load a preload file with the score file button. (see usage instructions for more info)

looping:

In loop mode the first two clicks in the score set the left and right edge of the looping range. (subsequent clicks adjust the closest range marker) synchronizing:

When your audio file is synthesized from score, no manual synchronization is necessary. Otherwise (e.g. live performance or video) you have to synchronize the media manually.
By clicking in a measure you synchronize the *first* beat of that measure to the audio.
Clicking in the first measure synchronizes the first beat (of the music) to (the first note) in the media file. This sets the, so called, offset of the media file.
The duration of the current measure and offset are shown in the top right corner of the display. You can precisely adjust these numbers with the keyboard sync commands (preferably when media is paused).