If it is unlocked you just press in the button. The trunk has 2 lock positions. Locked position one is when the trunk is locked with the other doors. Locked position two is when you lock the trunk from outside the car, meaning you turn the key past the point where the doors lock, this puts the trunk in sort of a deadbolt position and the trunk will not unlock with the doors. Your E30 has whats called central locking, meaning all the locks can be locked and unlocked from any of the 3 exterior locks with the key. If your car does not do this something is wrong with your central locking. This is a standard feature on the car, if it doesn't work it is broken.
Eample position 1: trunk will lock and unlock with the doors.
Example positon 2: trunk lock is put into deadbolt positon. Trunk can only be opened by putting lock back into normal position with key.