There is absolutely no way your car is still driving while being too rusted to lift properly. The car is holding it's own weight when it's on the ground, right? It only needs to hold it's own weight when up in the air, right? Your mechanic just imagines that your bleeder valves are rusted shut (and they might be), so he's using the lift as an excuse. Find another mechanic, preferably one with a lift where the wheels are being supported, like the lifts they use for wheel alignments.
It is way easier (for access) to bleed your brakes if you take the wheels off, and to do that you need to jack the car up. Be aware that jacking a car up to put it on ramps doesn't work the way you think. The suspension will stretch when you are taking the weight off of the wheels, you will need to lift the car much higher than your jack can likely handle. It is much easier to drive onto the ramps. I have ramps (built them myself, heh), I use them for everything except for things when I'd rather take the wheels off (like bleeding brakes). Every tool and technique has it's purpose.
That being said, I have a 94 Sentra (same body style as yours), so to jack it up (for this or other purposes) go find these:
Get behind the car, lay down on the floor on your stomach and look at the middle part of the rear axle, beyond the sway bar. You see that black round cylinder about 2 inches wide? That's the jack-point for the entire rear of the car.
Now run around to the front, get down on the floor on your stomach, see that black beam about 3 inches wide that runs from the radiator to further back than you can see? That's the jacking point for the front of the car.
Do NOT get under the car with only a jack holding it, put jackstands somewhere underneath. The part that usually rusts enough to freak out mechanics are the pinch-weld seams (the bottom of the rocker panels, that thing you gently kick to remove snow from your shoes before getting into the car). My mechanic friend always says to use those, but every time I do something bad happens (they crack or bend). So, now, go look for these:
Go next to your driver-side side-view mirror, get down on the floor on your stomach and look under the car for a beam running front-to-rear, about 18 inches away from the side of the car. There's a spot near the front of it (lined up with your mudflap) that is a little thicker. This is a jacking point for the car, there's another one on the passenger side. Sorry for the blurry picture :)
The rear is a little problematic on most cars if you don't want to use the pinch-weld seams. That beam you used at the front ends way too early to be used at the back. Don't use the actual floor of the car, that's very thin sheet metal and your jackstand may just go through it. Pinch-weld seams are your best bet, even if you have to move a little forward, if not there's this "mouth" just a little forward of the rear control arm pivot point.
If the underside of your car doesn't look like this, you don't have a 91 Sentra, you have a 91 Sentra Classic (previous generation)