Coming from 71 South freeway > exit central/soquel canyon and turn right > turn left on Elinvar > the road will eventually wind to the left, the entrance will be almost immediately on the right hand side (often times you will see many cars parked at this left hand turn because people like to hike inside) > drive through the gate and travel down the long road and you will eventually come to a state park booth where you can pay to get in with you car [I believe it's lime $5].
Jonathean nailed it
Off of Lambert where the Discovery Center is.
You can enter from the Orange County side off of 57fwy at Lambert. East on Lambert until it turns in to Carbon Canyon Rd. The CHSP Discovery Center is on the right about 2 mi after you exit the freeway. Parking is $5
There is another entrance behind the Green River Golf Club off the 91, near the Santa Ana River Trail.
Coming from 71, make a left at Elinvar from Soquel. Entrance will be on your right in about a quarter of a mile.
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