I took a trip from Portland down to Mount Pisgah yesterday, so here's how to get there from I-5:

From the North, I-5 crosses the Willamette River and then takes a dogleg to the East and then South. Take Exit 189, the first exit after the dogleg. You'll drive parallel to I-5 south for about a mile to East 30th Avenue. Turn left and cross I-5.

Next, turn left to head North on Franklin Blvd. Take the first right, go under the railroad tracks, then make another right onto Seavey Loop Road. It crosses Franklin Blvd., so if you miss the turn, stay on Franklin Blvd and then take a left onto Seavey Loop Road. Follow Seavey Loop Road. Seavey Loop Road turns south after a couple of miles. Don't turn. Just continue straight on Seavey Way until it crosses the Coast Fork of the Willamette River. Turn left. From there, I'd imagine there will be plenty of signs or just follow everyone else unless you're getting there very early like me.

From I-5 coming from the South, it's even easier. Just take Exit 189 which leads North onto Franklin Blvd., then follow the instructions in the paragraph above.

To return to I-5 North, go back along Seavey Loop Road, get onto Franklin Blvd. and go under the railroad tracks. At the intersection, turn right and follow signs to I-5. For I-5 South, turn left at the intersection, take East 30th Avenue across I-5 and turn left to I-5 South.

I'd suggest taking a look at Google Maps at maps.google.com/ and zoom in on Exits 188 and 189 from I-5. It's just a bit tricky coming from the North, but if you look at a good map, maybe print it out, it should be no trouble.