I would highly suggest using Waze or google maps to get to Merrick Hollow. This is really only crucial after a large rain because some of the roads they try taking you down are dirt and will turn to mud with all the rain! 
If you are coming from the OKC metro area, take I-35 north to Guthrie, Exit 157. If they confirm they have that route, it will safely get them there! 
If you are coming from the Tulsa area, take the turnpike to Wellston, Exit 158. Here is the important part, take a left to go back SE under the turnpike on Highway 66  to turn left (North) onto Highway 177 to go through Carney, NOT Wellston (all dirt roads once you pass through town). Turn left (West) onto Highway 105, and then your directions should take you from there!