There are DME -Disney Magical Express - buses available with lifts to lift a wheelchair into the bus. It is recommended that you request a lift bus when making your DME reservations if you will need one.
If you didn't prearrange it for your trip from the airport to your resort, they will still provide one, but your wait may be longer. If you need one when returning to the airport from your resort, it is vital to prearrange getting one. If you don't, you might risk getting to the airport on time. My family usually asks to leave for the resort extra early to allow for any issues - like a wheelchair bus not coming or the equipment not working.
Here is a picture of the DME bus lift in its highest position:
DME Bus steps
Someone using a wheelchair or
ECV can ride it up the lift and then the wheelchair will be fastened down to the floor of the bus.
A wheelchair or ECV can also be placed underneath the bus in the luggage storage compartment, but the person will need to climb the steps.
People have posted before that they were allowed to stand on the lift and ride it up for boarding because they were unable to climb the steps. This is no longer allowed for safety reasons after 2008, but you may see older posts saying they did this.
Only a person seated in a wheelchair, ECV or other mobility device is allowed on the lift. Manual devices need brakes locked. Power devices must be turned off.
No one may stand on the lift or sit on the floor of the lift platform.
If you have mobility problems or other disability, but can climb the stairs, the first row of seats is reserved for guests with disabilities.
If you feel you can't climb the steps, let the CM know when you are making your reservations for DME and again when you check in. They will need to come up with some alternative for you.
I have seen people using an airport wheelchair to ride up the lift, get out of the wheelchair to sit in a seat and then borrow a wheelchair at the resort to get out.
On our trips in the last 4 years, I noticed that there were 2 wheelchairs in the under bus storage area. Since we were the first on, the wheelchairs were on before the bus arrived at the stop.