I know this has been covered, but I just did it myself so now I'll explain it quickly.
1. use the stair tool, create the arc path of the stair using the run tool.
2. finish sketch.
3. turn off the visibility of the stair. Leave the handrail visibility on.
4. creat a rectangular profile(the shape of your ramp, ie 4" deep 3' wide)
5. load profile into project.
6. pick on handrail(note you can delete on of them or change it)
7. edit the properties of the handrail to contain the rectangular shape at 0 feet above the ground.
8 add or edit the rest of the railing as desired.
9. look at 3d and say wow, 3d curved ramp.
Note you can mess around with the rise and run and level setting to adjust the slope etc.