Dalles Mountain Road is on the Washington side of the Columbia River. The turnoff is exactly one mile to the east of the junction of Washington Hwy 14 and US Hwy 197.
Immediately after the turnoff, the road forks. Take the right fork. Follow this road until you come to a group of farm buildings. At that point, you are in the middle of Dalles Mountain Ranch and Columbia Hills State Park.
If you wish to, you can follow this road all the way to the Goldendale Valley. The road can be rough in patches, but the reward is some spectacular views of the Columbia River Gorge.