Any mark by anyone one always applies a -2 penalty to attack rolls if the attack does not include the creature that applied the mark.
If the monster's power (or PC's power for that matter) does not specify anything else but that the power marks the target, then you just apply the -2 penalty to attack rolls.
If there is something else that happens (divine challenge for paladins, combat challenge for fighters, etc), then the power will say so.
(see conditions on PHB page 277...Marked is a condition an has it's rules in the chart).