Trying to locate where to put gear box oil in my 1999 hyundai excell

what tranny? 5sp manual.
only the 5speed uses, real GL4 gear oil. so. must be.
under car, look up. see box. on right side is the fill hole.
seen here.
here is my filling mine before , new engine install. can be done
with engine, using free gravity and hot lube.http://www.fixkick.com/tranny/GL4-easyway.JPG
under car view.
http://www.fixkick.com/tranny/custom-oil-filler1w.JPG
its also in the glove box, operators guide.
and online at all majors, for that, alldata dot com covers it best.
http://www.fixkick.com/IGN_timing/tune-ups.html#Gear_Lube

There should be a level check/fill plug about half way up on the transaxle, about even with where the axle shafts go into the transaxle. Take the plug out and refill with oil till it just starts to ooze out the hole, then replace the plug.

There is no stick to measure a gear box. You check it by removing the filler hole on the side of the transmission and sticking your finger down in it. Usually you fill it until it starts to drip out, then you know it's full.

HI. WELL I DON'T MEAN TO BE A SMART *** BUT IF THAT FIXED IT BEFORE WELL REPLACE IT AGAIN.

try the left side of the engine... gear oils smell are diffent from the engine oil and the viscosity... gear oils are thicker... u wont go wrong on this...

Go to http://www.hmaservice.com/ and register (vehicle by VIN). Afterward, you'll have access to technical information (manuals, wiring diagrams, service bulletins, etc) about your vehicle.

sounds like you are low on gear oil

Have your oil change at another shop.See what they say the problem is.

The fuse box is located low on the left side of the dashboard behind a cover, or at the left side kick panel, behind a cover.

Relay Box -is located under the left side of the dash as well.

I'd get second opinion from transmission repair place, but it does sound like an inspection is in order. Nothing against your mechanic, but transmission places are more experienced in these matters, and it may save you time & money in long run. Remember, an estimate does not mean you have to commit to repair right then & there.