how to see a play at the colliseum?

It is very unusual to see a play at the Colosseum. From time to time big celebrities like Andrea Bocelli, Paul mcCartney etc have given concerts there, but I have not heard of any actual theatre going on there. What you may want to look at is the summer concert and cabaret series just behind the colosseum, which does overlook it. Here is a link for the calendar to give you some ideas of the types of performances.
http://www.allombradelcolosseo.it/index.php?option=com_eventlist&view=eventlist&Itemid=63

I've never heard of plays in the Rome Colliseum. There is a Roman arena in Verona and all spring and summer there is opera in the arena. You can buy tickets on site.

The Miracle Players usually perform a play in English every summer at the Forum and it's free but I don't know of anything taking place inside the Coliseum.

http://miracleplayers.org/

The Colliseum is a ruin. It's really not capable of supporting much inside. When you get there, you will see what I mean.

The ampitheater in Verona is still very usable and has opera.

We nearly skipped the ampitheater in Verona because we would see the Colliseum later. I'm glad we didn't skip it, the working ampitheater gives a much better idea of what it was like during Roman times. Fortunately, it's made of stone that wasn't worth quarrying so it remains intact. The marble that was used as a facade on the Colliseum was taken as building materials for other projects around Rome, I believe including St. Peters.

Edward,
There were no plays in the colliseum when I lived in Rome however you can see an opera in the ruins of the baths of Caracalla. The operas are very well done even if you can understand them (but neither can most of the locals)

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Terme di Caracalla - Rome Opera