There is a special section in the stadion where you can access and see the game. However, it´s quite hard to get tickets because there are few spaces and the tickets are in high demand. It will depend on the game.
I went on a game at Camp Nou, but it was many years ago, and thus I don’t know if my experience can help you.
We were three power wheelchair users and one manual wheelchair user (with our personal assistants), and we went to the stadium on the same day in morning (the game was in the evening). We managed to buy three special tickets for the power wheelchairs, and five normal for the others (the manual wheelchair user could transfer to a normal seat). I know that the stadium was recently renovated, but in those days there was plenty room for wheelchairs.