To get to MI, there are two ways. 1) Get up to Finch Stn, then take the 77 YRT to Bramlea (get a transfer) from there take the Dixie Rd south (with said transfer)
2) Get to Islington Stn, take the Mississauga Bus to Dixie and take the Dixie bus north, but this may only take you 0.5km south of Midway Blvd so you will have to walk.
Your going to have to visit the respected transit websites to see which route would work best. Either way, you may have to take a taxi back.