gluii
Try this:
1) Set a boolean variable to "true" indicating when a building has been entered. This can be a global if you only have one building or if you have many buildings try with an instance variable for each one.
2) Set up a condition checking for the value of that variable, if it is "true" then do what ever needs to be done to show the inside of a building. If it is "false" then you do what needs to be done to show the outside of the building.
3) Set up a condition block to set the variable back to "false". This one should only work, when you are on top of the door AND you press the appropriate keys (maybe the up arrow?) AND the variable itself has a value of "true". That way you can be absolutely sure it will only be turned to "false" at the right time.
What ever you choose to do, you might run into the problem of entering and exiting very quickly out of the building as long as your character is on top of the door. So you might need some additional conditions to make sure that the actions to enter/exit the building are not executed in rapid succession.
Hope that helps.