Pandemonium is an enchantment that says "Whenever a creature enters the battlefield, that creature's controller may have it deal damage equal to its power to target creature or player of his or her choice." Muscle Sliver is a 1/1 creature that gives all Slivers (itself included) +1/+1.
So if you have no other Slivers in play, does Pandemonium do 1 point of damage or 2 points of damage when Muscle Sliver comes into play?
Pandemonium's triggered ability checks the power of the creature when it resolves, not when it triggers. A ruling on Pandemonium says the same thing.
The triggered ability does not check the creature's power until the ability resolves. If the creature is not on the battlefield when the ability resolves, it does damage equal to the power of the creature right before it left the battlefield.
By the time the ability resolves, Muscle Sliver is already on the battlefield, and his power is being modified by his own ability. You may have it deal 2 points of damage.
Still not convinced?
There are cases when a creature's power is checked when an ability triggers and when it resolves. To illustrate this, consider the case where we play Muscle Sliver when we already have an Experiment One on the battlefield. Will Muscle Sliver trigger evolve?
The answer is yes. This is because of rule 611.3c (although it is mentioned in a few places).
611.3c Continuous effects that modify characteristics of permanents do so simultaneously with the permanent entering the battlefield. They don’t wait until the permanent is on the battlefield and then change it. Because such effects apply as the permanent enters the battlefield, they are applied before determining whether the permanent will cause an ability to trigger when it enters the battlefield.
Muscle Sliver will deal 2 damage to a creature or player due to Pandemonium's ability.
Pandemonium's ability is a triggered ability that triggers when a creature enters the battlefield. So when Muscle Sliver enters the battlefield, the triggered ability is put on the stack and you choose what creature or player to target. At this point Muscle Sliver is already in play, so its power is 2 due to its own static ability. Then the triggered ability resolves, and Muscle Sliver deals 2 damage to the ability's target.
