1. Read snYpir's tutorial "How to use objectives and init.sqs." A good general tute will probably help too. Conceptually, objectives are associated with triggers. The objective is expressed as the trigger conditions: when the conditions are met the trigger fires and the objective is completed.
2. Place the loon on top of the tower in the Mission Editor. In his init field write
this setpos [getpos this select 0, getpos this select 1, (getpos this select 2) + 6]; this setUnitPos "up"
You'll have to play with the 6 until you get the right number.
For all noobs, I recommend you read snYpir's OFPEC Guide: making missions and my own macguba's guide to mission editing for beginners. Both have a lot of useful tips including suggestions on other good tutorials. All available in the Editors Depot.
And in case nobody has said it yet ...
Welcome to the forum!