I decided to do some editing, and created new stadiums for all English Teams, so I would type in the capacity, change the seat colour, assign the stadium to each team etc...however when I select the stadium it says "file not found" and when I play I just get a football pitch with no seats or terracing.

hehehe... you can't simply create a stadium like that. of course you have to build the stadium (or download it) and then you can import it with creation master.