Summary - Roughly two hours from Portland, getting Mann Butte includes a steep bushwhack through heavy brush and deadfall and ends in a small, tree and brush covered summit. You can see Mt Adams through the trees but not much else. While you can drive to the base of the summit, if you value your vehicle's paint, you will hike the road in. Good tires and 4WD, long pants and sleeves, map and GPS or compass are all really good ideas. Bring HF gear as 2M contacts with an HT are difficult.
TL;DR - Unless you like fighting your way through saplings, brush and deadfall to a largely view-free summit, there are more enjoyable activations to do in this area (for example, Little Huckleberry). You will get a peek at Mt Adams from the summit and there are plenty of trees for putting up wires (though no open space). I had little luck on 2M, even with a yagi. ATT Digital cell service was good as was two way APRS. After working through the HF bands, I saw no reason to linger, and the flies had found me, so I packed up and headed for the truck.
To get to Mann Butte, take WA141 through Trout Lake to FR24 to FR41. Head south on FR41 for 1.4 miles and make a right at the intersection; another mile brings you to FR061, follow that to (45.94, -121.64). You will find a road here that curves around the west side of Mann Butte. Turn here if you don't mind demolishing the paint on your vehicle, otherwise park and walk. After reconnoitering around the base of the summit, I went up at (45.94, -121.64) following the ridge to the summit. Stay to left on the ridge and you'll avoid a saddle between you and your destination. Set a GPS point or be good with a compass as the terrain in the first half of the descent will encourage tyou to come down east of the access point. The good news? The road wraps around the butte, so it is hard to miss.