"Q- what type (weight) of oil do I put in this transmission?"
I guess you don't have an owner's manual? Even browsing through a Haynes manual might give you the spec of the a fluid. Doesn't appear to be a lot of Stanza peeps out there discussing this car.
If you need the car for a few days, a 30 weight motor oil should work ... but you'll want to drain that out ASAP if you find that the box is supposed to have a gear oil like a 80W90 gear oil. Gear and motors oils are "weighed" on a different scale so the differences in weight aren't as great as you might think at first.
There
are a lot of differences in additives, however, so you don't want to leave a motor oil in for long if it calls for a GL-4 or GL-5 gear oil.
"Q2- Where is the fill plug? I looked and looked but can't find where I would put it in ..."
Should be about 6-8" above the drain ... but is rarely
directly above it. Often it's a little further forward or back.
You'll need a funnel and a chunk of hose (1/2" or less O.D.) to refill it ... oh, and a lot of patience. Stuff goes in slow.
Most synchro boxes take about 2 quarts. Add the 2nd (or possibly 3rd) quart slowly and watch for fluid weeping out of the check/fill hole to show you that it's full.
On some cars, you can also fill it where the gear linkage goes into the top of the case ... but you'll have to guess at capacity without a manual.