As you haven't indicated the model/year of your car, be aware that the parts and process for the M57 are different from the M57TU (pre-facelift and facelift).... (I know yours is an M57TU, Lamby).
Firstly, you are going to need to remove the fan in order to remove the v-belt belt and tensioner.
The tensioner removal process for the M57 is here: -
BMW Workshop Manuals > X Series E53 X5 3.0d (M57) OFFRD > 2 Repair Instructions > 11 Engine (M57) > 28 V-ribbed Belt W.tens.-deflect.elemt > 2 RA Removing And Installing Alternator Drive Belt (M57 _ M47TU) > Page 525
The same process for the M57TU also looks to require the removal of the alternator: -
BMW Workshop Manuals > X Series E53 X5 3.0d (M57TU) OFFRD > 2 Repair Instructions > 11 Engine (M57TU) > 28 V-ribbed Belt W.tens.-deflect.elemt > 2 RA Removing And Installing Alternator Belt Tensioner (M57TU)