You have FW options also, but if you specifically want MS:
http://www.mohegansun.com/getting-here/bus-ct.html
There are 2 options. Dattco or Greyhound.
I've used Dattco before with no issues but that was always chartered trips, not the normal line runs.
Dattco has 3 schedules. The Daily-AM has a pickup right at the Ferry (For $40 vs. the normal $28)
You get vouchers for $25 or $30 worth of food/slot play. Not sure if you get anything else if you pay the $40 from the ferry.
All 3 routes have a pickup in Stratford so you would need a few minute cab ride if you were going on the Daily-PM or Fri/Sat late night, or didn't want to pay the extra $12 from the ferry on the Daily-AM route.
I'm not familiar with Greyhound. I know there is a pickup in Bridgeport, just not sure where.
And I say this half kidding, but if you are not familar with Bridgeport, don't wander off with the wrong colors