No, Illuminate just increases encounter rates. You're more likely to see a Pokémon in the wild if a Pokémon with this ability is leading, but no way to control exactly what. It's similar in a way to Sweet Scent's usage in drawing a wild Pokémon to wherever you are at that moment, except that Sweet Scent is like battling something upon request instead of wandering.
Mean Look is a definite learn later on. It is vital to help you get Tornadus or Thundurus. It always runs on the first turn and you then must relocate it. You CAN ignore it (it does first appear before the E4 when a Gatehouse worker calls you over to tell you about it as you head to Route 10, and the entire roaming event is triggered with a trip to Route 7 to talk to someone in a house there...I won't spoil the event itself), but once triggered, it does roam random Routes in a set fashion, and can be tracked in the Gatehouses. Before you defeat Ghetsis, the random Routes during overnight hours make it uncatchable (Routes 12-15 according to my research), and it appears in the lower Routes during the morning, and roaming in your training grounds during the evening hours.
Back on topic...Tornadus/Thundurus always runs as its first move, much like an Abra uses only Teleport (and I'm having issues catching said Abra, Mean Look is hard to come by in Emerald early on). The battle strategy once you trigger the battle sequence with it is to use Mean Look, then make sure that Watchog does NOT faint (though never let your target run out of moves or you'll fail). It can't lower its own HP (such as with Take Down), so the second move to use is Hypnosis, then spam Super Fang once it's asleep, putting it back to sleep as necessary, and also healing (bring Hyper Potions if your Watchog is a high HP one, though Moomoo Milks from Driftveil might do the job well enough). Also, I'd try to level your Watchog to at least Lv.45, even Lv.50 if you want to take the time before going to the final Gym, as the higher leveled it is the more punishment it can tolerate.
Once you've spammed Super Fang a few times (to the point it has maybe 1 HP left, since it can never take the last HP), start chucking Pokéballs or other types of balls. Don't waste your Quick Balls, or a failure will just make it run since it's only best on the first turn. I think mine ended up in a Great Ball after some failed Ultra Ball attempts. Use whatever works, basically, and even if you run out of Ultra Balls keep chucking something. If it's nighttime, Dusk Balls could do the job as well. :) Just remember, if Watchog faints, your target gets away. The good news is when you find it again, it's exactly how it got away (1 HP, sleeping, etc.), but why waste the time? ;)