Mercenary eggs are a tricky business. For one, always know that a Dropped Merc egg is NEVER a guaranteed hatch. The Kunemon and Drimogemon eggs you get are quest eggs and there for will always hatch to 3/5. (Pay in mind, having a egg hatch at 4/5 is the preferred route.)
In DMO, our version at least (The JM ?Version) We have a lovely little thing called a 'Drop Cap'. This means if you are 7-8 lvls higher than a digimon, you get nothing. No drops aside from some quest or event items.
How, as for Merc Egg drops. First check around and ask if a certian Digimon is capable of dropping a Merc egg. Then, at that point it is pure, blind, randomized luck. Mercenary DigiEggs are RARE drops. There have been cases of folks, even myself, who spend hours grinding on a certain set of Digimon and never saw a single egg drop. And the opposite when we've acquired several within a few minuets.
Just be patient, keep killing, and don't give up hope. Just remember, it's all Luck.
You'll know when one drops, as they will appear as large, spinning eggs with an enormous glow around them.
Remember, not all Digimon drop Mercenary eggs.