Duple click the stair tool and in the panel "preview and Positioning" click on the single arrow next to the name of the stair and choose "Create new stair"
In the new window "Stair Type Selection" if you look on the top you can see 2 buttons,one for a usual stair and one for ramp.
But i don't know if you can use all predefined types for a ramp ,additional to this there are some add-ons such as ArchiStair or StairBuilder(i am not sure about the second one)
There is also a library item named "Curved Ramp " in the structural elements folder, try this too.
