Turn right out of the parking lot, go to the first light (Dixon Road). Turn left. Dixon changes it's name into "Airport Road" Follow it as it winds around, past the Airport terminals, past Zet's (great greasy spoon), and up to Derry Road (a big main road). Turn left onto Derry Road and go west. Watch for Threshold on the right hand side. It's in a little business park
Or, look it up on Google maps: maps.google.ca "threshold aviation" city "mississauga"