How far is the nearest station? :?
The nearest station is Thirsk, but from what I know / have just researched you will need to get a bus to Ripon, and then another to Lightwater Valley.
Depending where you are travelling from, and what your preferences are I think that you would be better getting the train to either Leeds or Harrogate and then the number 36 bus to Ripon. (Harrogate has the advantage of the bus station being close to the rail station)
I'm very tempted to join you guys there now. :D