The ADA doesn't apply here. I know a fair bit about this subject, because I do ADA/FHA accessibility assessments for HUD and universities. The ADA Title III applies to areas of public accommodation, like the leasing office, the pool if it's open to members' guests or public events, or community rooms if they are available for rent to the public, plus all of the accessible routes and facilities related to these areas. The ADA does not apply to an HOA member's unit or the route to that unit.
However, the Fair Housing Act does apply. The FHA accessibility requirements apply to the individual units and the routes to them. But there's a big caveat. The HOA member can make a request for an accessibility feature, but the requested work has to be "reasonable" in order for the HOA to be on the hook to pay for it. If the work represents an undue burden to the HOA, either financially or operations-wise, then the HOA can require the member to pay for part or all of it, or deny it.
Also, the FHA accessibility requirements apply to properties occupied after 1991. For anything built before then, you are at the mercy of what the HOA considers "feasible". If they don't deem it economically feasible, you're out of luck. For example, there are thousands of multi-family buildings in NYC that were built prior to 1991, and none of them are required to meet FHA requirements because it's simply not feasible to do so.
My guess here is that a ramp seems reasonable if you can demonstrate a permanent disability and the only way you have to get in and out of your unit and to your car or public transportation is by ramp. It doesn't hurt to make a request. But if your neighbors have seen you walk around and go up and down steps on your own, it might be a tougher prospect.
