Greetings!
I, too, had issues when I first started using trampolines. Let's see if I can clarify it a bit.
So, on destination B you have a trampoline on COORDINATES (this is important). On start point A you have some fleet, say a few cruisers and a trampoline, also on coordinates. lets say you have some cargos there for fuel as well.
Find the fleet on point A and click "sending". On the fleet page, you will select your cruisers (and cargos, if necessary), BUT NOT the trampoline on A. One trampoline must receive the fleet, the other must remain stationary to send them (the one on A).
Send them to COORDINATES B on ACS D mission. Now, look at the bottom where the "SEND" button is... at this point, you should have another button "Sent thru trampoline". You should also notice that the travel time and the gas used will have two values.... one if you just "send", the other in () if you use send thru trampoline. Change your speed values accordingly.
Click send through trampoline, and your fleet should be on it's way :D