Ok, jet dry isn't a detergent, it's a rinse aid. It helps dishes rinse and dry off without any spots. Unless your washer has a specific reservoir marked 'rinse aid' that you can open up (often you have to unscrew it) you don't use it. You fill up that reservoir completely and the washer will squirt in a little at a time over multiple loads. You don't have to measure every single load and a little goes a very long way as you've found. Anyways, it's not necessary, it just helps the dishes look a little nicer (no water spots) so I probably wouldn't bother because it seems like you have bigger problems.
Go down to walmart, target, or your grocery store and go to the aisle with the dishwashing stuff. There will be powders in boxes and liquids in bottles. Either one works well. Personally, off brand dish detergent is very hit or miss. I recommend just getting a box or bottle of cascade. It's usually relatively cheap and works very well.
Load up your washer (in general plates and things on the bottom, bowls and stuff on the top) and then look at the inside of the door. There should be a reservoir marked for detergent and maybe one marked prewash. Fill the normal one up and put some in the prewash if its there (you don't really NEED any in the prewash, I just tend to toss a little bit in there. Close the latch over the normal detergent reservoir, shut the washer door, and start it up.