To install a smoke detector in a suspended ceiling, first take down the tile in which you'll be inserting the detector. Find the screws that came with the smoke alarm and the toggle bolts that go on the screws. The latter are what will hold the smoke alarm in place.
Drill holes in the tile the same size as your screws. Fasten the smoke alarm's mounting bracket to the tile using the screws and toggle bolts. Once everything is secure, hold the test button on the smoke alarm to make sure it works and then place the tile back in the ceiling.