If you do what mckill stated, you are done with your problem.
Basically what that trigger does is when you select a unit, a variable of unit is set to that unit.
And another trigger will fire off that will lock your camera to the last selected unit, you selected (mouse-pick / drag-select).
But that will work for only one unit, if multiple of units, how do you want to lock all of them ?
It makes no sense.
mckill, try add an array (edit the post) to the variable (Player number of (Triggering player)), that will become MPI, perhaps his map is multiplayer ?