How can i use my created stadium?

I created a stadium and applied it to PES United (who I re-branded into my local team-Chester FC). It was available towards the end of the list of stadia. TBH, the animation sequences for custom stadiums aren't really that great.
It's also possible that you can't change the stadium of teams such as Manchester Utd, Barcelona etc, who have their licensed stadium on the game.
Try a few different things and see what works.