[Michael Romanos]

Climbing is a natural thing for Corgis to do - whether the climbing includes man made steps or natural steps in the forest, bush and hills. If there is some kind of mental difficulty, I would treat stepping up stairs like any other training procedure. If a Corgi is free running in a park or on a track and steps are included, I have yet to see one who thinks twice about racing up the steps - unless there is a upwards straight run on the outside of the steps.
Go to an outside venue which has one step or two (maybe the entrance to the front or back doors of your house, and on or off a leash, lead your Corgi to the first step and offer a treat to coax him up the step, maybe by placing the treat far enough away so that your Corgi has to step up first. When he performs the action and gets the reward, praise him as well and start the process over again several times. Obviously you would progress to two steps then a whole rung of steps. I also think steep hill climbing would do wonders to get a Corgi into a stairs climbing frame of mind. Corgis can go up really steep hills especially with a fast start on the flat beforehand.