I don't know what you mean when you say pop off the plastic cover (might be one of the later ZF boxes) but on most, the oil filler is the square headed plug next to the adjuster. If the adjuster has a couple of threads showing above the top of the lock nut, the box probably has a lot of life left in it. When well worn the adjuster is screwed in much further and maybe no threads are showing at all. If the box is still "smooth" that's not a problem, but long term you may want to look out for another with less wear. Adjustment is down to common sense but a little too loose is far better than too tight. Make a note (picture) where the slot in the adjuster is before you have a go at it so you can put it back as it was if needed. A bit of slackening and tightening will show you the effects better than all of the book and posted descriptions.