It is easy to access, not behind any barrier or guarded by anyone. It's just on the right of the car park which is just a right turn off the main road back to Lymington.
It is a bit more complicated than that!
It is located on the west of the Wightlink terminal. You have to be careful on exiting, (only at the southern end), not to get run over by traffic boarding or alightling from the ferries.
There is a small cafe in the Wightlink terminal. You would be best leave the site on the extreme east since traffic regular users the western side. It would take you about 20 minutes to walk back to the Town station.
It has sadly changed over the years. Was once a very interesting set up in the '70's and I remember the summer saturday direct services to Waterloo.