If your other lifts are good, and you can't do a pullup, focus on those first in your workout. Of course assisted lifts aren't "as good as the real thing", since you're not lifting your whole body weight I think they're a decent way of working up to pullups though.
Stick with the compound movements and their variations. Adding pull-ups will obviously train your back tremendously. The invention of the assisted pull-up machine is to allow those the opportunity to perform the exercise with a lower percentage of their bodyweight until one is able to achieve this using full body weight. In my opinion, assisted pull-up machines are great tools that allow the lifter to properly understand body alignment, path of motion, gauge strength levels, muscle activation and proper force-coupling of muscle groups that allow the execution of a good pull-up. You also have to ensure good postural alignment through stabilizing (isometric) strength whether you have to stand or place your knees on a platform.
I disagree with the previous posters. Forget the assisted pull-up and dip machines. Do the actual exercises first in the workout, when you are strongest, and if you are too weak to do a single rep in each, do negatives. Jump to the top position and lower yourself as slowly as you can. This should build your strength nicely. If you are strong enough to do about 3 pull-ups, do sets of 2. About 4-5 sets should do the trick. When you are strong enough to do 5 or so, do ladders. This technique means to do one rep, rest, do two reps, rest, do 3, etc, until you hit your max, then drop to 1 again and move up again. It is psychologically quite easy in the early sets, and a good way to get a good amount of reps in. I have had success this year coaching athletes using these methods, including some fat rugby players who couldn't do a single chin up in April, but now are approaching ten.
Good advice above, hit them when you are fresh and eventually in conjuction with your other basic moves and good nutrition you will see your chin reps increase. Always practised my chins and many other training partners i have trained with have said "I'm too heavy to do those blah blah" just got to practice and having good grip strength helps as well. Just done my best set of chins last week with six reps with a 3 sec lowering and a 20kg plate between my legs, now i need to buy a weight belt as can't get any more plates now between my legs! Good Luck
In my experience, yes, assisted chins and dips helps a lot. The best thing for these is to of course lose weight.
Deadlifts are a big winner too (for chins). I'm so glad I've started doing these, thanks to all the good things said about them on this site. The extra grip strength and general pulling strength really helps.
Whether you go with assisted or negatives, once you can do a single dip or chin on your own, I'd ditch the assisted entirely. Just start doing multiple sets of 1 (increasing the reps when you can), with slow negatives, and your numbers will start to go up quickly.
A couple of months ago, I could only do a single chin, but now I'm up to 8 straight or 4 sets of 4.
I do the whole routine every 48 hours. However, I alternate squats and deadlifts each time because I feel like I need more recovery time for those. So Monday I'll do squats, bench, military, rows. And then Wedneday I'll do deadlifts, bench, military, rows. Rinse and repeat. On my off days I like to swim for about 45 minutes so I can actively recover and stretch the muscles.