The full address is ...

51 Union Street, Oxford, Oxfordshire, OX4 1JP

It's not that easy to find. Luckily the former tenant's direction are still available at  http://peopleandplanet.org/dl/directions.pdf

The original article was posted on Oxford Indymedia, so I assume the poster didn't realise the post would be seen outside of the Oxford context. There's not a lot us moderators can do, other than trying to offer clarification when we can - hopefully this has helped.

Plebs' college is currently at:
51 Union Street, Oxford, Oxfordshire, OX4 1JP

This shows the exact spot geographically, but doesn't show how to get there from Union Street itself:
 http://www.openstreetmap.org/?mlat=51.74889&mlon=-1.23466&zoom=17

To make it a bit clearer, see the attached satellite picture, with labels.

Directions:
1. On Union St, look for the set of green metal gates next to East Oxford Primary School.
2. Go through the gates, up the hill through the car park.
3. Plebs' College is on the left. A sports pitch is on the right.