From Indianapolis: Take I65 north to the State Road 26 exit 172. Turn left and follow St Rd 26 to Sagamore Parkway (US 52). Turn right onto US 52 and follow it to the first stop light. This is Union Street. Turn left onto Union and follow it through Lafayette and over the Wabash River. This will turn into Fowler Ave and eventually Northwestern Ave. near campus.
For more detailed information on the Purdue Campus, see the Campus Map.