welcome to Snow and Mud. First off the area referred to as the dunes means different places to different ppl. So I will tell you both of them.
My idea of the dunes
From Rycroft head south on hwy 2 to Grande Praire, turn left on 116 and get into the far right lane ASAP and take your next right on 99st (resources road) follow that road out of town and about 4km outside of town you will see the dunes golf and country club on you right. Turn left instead and cross the train tracks and you will see a staging area on you right.
The other.
From Rycroft head south on hwy 2 to Grande Praire, turn right on 116 ave (hwy 43 by pass) follow out of town for about 6.5km and turn left on range road 62 (FYI this range road 62 will also bring you across to the dunes golf and country club) take the right 50 meters in on RR 62 follow for about 1km and you will see a sand pit on your left. You can park there or keep following the gravel road down the hill around another pit and there is a left turn into the main part of the gravel pit and park in there.
There are allot of trails and it can be easy to get turned around out there so I would suggest you go with someone who knows the area or pack a GPS.