caz, if the cup is filling up with with a dark brown water to a dark brown sludge, it's working.
One thing to consider is how long has your tank been setup and how much livestock is in it? If it's say, a 55gal tank with 1 damsel in it, and it's been running for 1 month, the skimmer won't have much, if anything to skim out of the water.
But, it took my skimmer about 4 days to start making some skim mate, however it took my dad almost a week and a half for his to start making anything, and closer to 3 weeks to start making a lot, because he tank was so new and understocked.