I have one of these and it is a bit fiddly.
You need to decide whether you are going to count up or down and select that.
Set the alarm limit. I use 8mins 30 secs; this normally gives me an extra 5 minutes at medium power. The reason for this is that sometimes we have pedestrians under the flight path and we have to wait for them to leave before landing.
The throttle positon sets whether the timer counts and you have to set the throttle lever, then select "On" in the menu window.
If the throttle is closed, the timer doesn't count, but when you open it above the trigger point, then it goes.