Firstly, spark plugs on your ML320 should be changed every 90-110,000 kms (55-70,000 miles) or every 4 years, which ever occurs first.
To replace spark plugs you will need to remove air filter housing assembly, all coil packs x 6 and all leads x 12, just make sure you mark/note each coil and lead position.

Removing the leads from the spark plugs is the hardest task, holding the lead end (metal part) twist or rotate at the same time pulling hard, whatever you do don't pull on the lead!!
Their will probably be a lot of sweat, blood and maybe tears happening at this stage, oh, and usually a lot of swearing/cursing, so make sure there's no minors around.

Just take your time and be prepared to spend a few hours performing this task.