From 293 South I usually take exit 6. If you are coming fomr the south you would probably want to take exit 5 although their directions are only given from exit 6. Here is a link to the Vermont Salvage website with a map and directions to their Manchester location:
http://vermontsalvage.com/manch.html
And, as they say, dress appropriately, the building is unheated. The lighting is also not the best so a flashlight can sometimes be a big help.
Happy hunting!