Network Colchester bus number 8 as well as First bus numbers 61 & 65 head towards the store. You'll want to alight the bus at the end of Mill Road. After that head over the roundabout and follow The Cresent round to the next roundabout. Head over that roundabout and the store is through the yellow gates on the right (just behind Chef's Eating House).
The nearest train station is Colchester Station (or Colchester North Station, not Colchester Town). The station is around 3 miles away, but bus number 8, 61 & 65 mentioned above stop at the train station. Alternatively taxis are always available from the station (fair is £8-£10 each way, ask for The Seedbed Centre).