Hopefully you're only missing the cap. If so, you should see six allen wrench bolts that affix the lock hub to the wheel hub (circled in red in the illustration below). Also, the drive shaft end is recessed about 3/4" within the lock hub housing.
Auto Hubs.jpg
If there's no lock hub, the drive shaft will protrude outside the wheel hub and you'll see a ring with 12 holes, four with phillips head screws, surrounding the drive shaft.
Auto Hubs_1.jpg
If the hub is not unlocking, the Wheel-Lock light in the gauge cluster will not turn off. You can also put it in 2wd, use the unlock the hubs procedure, and then raise the front wheel in question. When you rotate the wheel, it's drive shaft will not turn.

If you're just missing the cover and it doesn't look filthy inside, flush it with WD40, push new grease in as best you can and screw on a replacement cover. This will probably be fine, especially if you found it engaged/disengaged per above. If there's a lot of grit or contamination, removal, disassembly and cleaning is recommended. If you're not comfortable doing that, best to take it to a reputable shop.

The Aisin hub is the only manual one I know of that's still available. But look at the 4x4wire.com and pirate4x4.com forums. Some members have raiders and monteros, and may discuss hubs other than the stock and Aisin brands.