I haven't actually had to do this one myself, but I remembered reading about it on the Construct Wiki.
Here's the page if you're interested:
and the relevant excerpts:
Setting a time scale
Use the system action 'Set timescale'. A value of 1.0 corresponds to normal time. A value of 0.5 means time in the game passes at half the normal rate.
Set the time scale to 0 and the game will come to a halt, since game-time is frozen. However, the framerate is not affected, and events continue to run. This can allow you to control a pause-menu or allow other commands to be carried out while the game is paused.
Seems pretty easy! With your newfound menu skills you can probably come up with a decent pause-menu as well.