Material count is the total value of pieces left for that player. For ICC the individual piece values are: Queen=9, Rook=5, Bishop=3, Knight=3 and Pawn=1. The total you start with is 39.
BlitzIn interface: Menu above: Edit > Settings > Board/Pieces > check Material Count
It is shown behind the username and rating in between ( ).
Dasher interface: In the blue bar above the board it gives, behind the username of players, in between [ ] the material count.
ICC for Mac/Windows interface: There is no option yet for material count. You can see the pieces a player has lost behind his clock.