Hi Wendy: I had the same problem when I reenameled mine.
Those things usually are threaded through the tub drain opening into a drain flange that immediately turns 90 degrees and ties to your 1 1/2" drain. This assembly should have an access panel to get to it. However, this access will not help you with your drain flange removal.
Plummer's supply houses do carry a tool with wide fingers that goes down into the flange then spreads apart and tightens out against the sides of the flange, which you then turn counterclockwise to unscrew the flange. However, I removed mine by using a pair of small (and inexpensive) pliers which fit down between the drain's cross bars. Once firmly in place, I put a pipe wrench on the pliers down as close to the flange as possible, then put a 2 foot piece of 2" pipe onto the handle of the pipe wrench (called a 'cheater') and with all of this leverage, broke the drain flange loose and then untreaded it. I made sure to put a thick blanket in the tub before doing this to keep from chipping the tub's enamel. Normally you would remove the flange (and all tub hardware) before re-enameling the surface, as you will most likely chip the edge of the new enamel while removing the old flange. If you elect to go ahead and remove the flange and replace with a new one, be sure to reseal the new flange with plummer's putty.