New Bike Paths!
The shortest route to the Arb is along West Main Street (M43) and it's now much easier due to Oshtemo Township's new bike paths. Both sides of the road! The only problem now is getting across the M 131 interchange - but that's manageable, with patience.

For a longer but nicer alternative Joe Renze suggests the following:
Follow West Michigan Ave through the WMU campus (the road stops for a couple of blocks and then starts again on the other side of Western). West Michigan Ave will eventually turn into KL Avenue. Stay on KL until 4th St.
Turn right on 4th and keep going north until you reach Almena.
Turn right on Almena and continue to W Main.
Turn right on West Main - stay on the shoulder and go maybe a mile until you reach the turn off for the Arboretum - a narrow, paved road leading up to the right.
This is 2-3 miles out of your way, but a nice ride. It takes between 45 minutes and an hour 15 minutes for an average rider.

Going back you can turn right on West Main and stay on the shoulder until 8th St.
Turn right (South) on 8th street and continue to KL Ave.
Turn left (East) on KL and continue back to the College.
This route is much shorter ( 30 - 45 minutes) but still avoids the bad part of West Main.

Note: Bikes are not permitted on the Arboretum trails.