You may have two problems...a failed diverter and a leaking drain.
What type of shower/tub diverter do you have?
If it's a diverter on the tub spout, replace the tub spout with one just like it.
If it's a diverter on the wall, it may be leaking there, and you'll have to repair that. Depends onthe type of faucet and set-up that you have as to what exactly to do.
You're going to have to remove the insulation to find out where it's leaking. Have someone stand in the shower/tub and run it so that someone can see where it's leaking from underneath with a flashlight. Sometimes you can only make a drain leak with the weight of someone standing in the unit.
You may have a drain leak, as well as needing to replace the diverter.
If it's the leaking around the drain in the tub, that's an easy fix, also. Just heat and soften the plumber's putty that is supposed to be sealing under the lip of the drain with a hair dryer, stick the handles of pair of pliers down into the cross-pieces of the drain, and unscrew it counter-clockwise with a screwdriver in the teeth of the pliers for leverage. (Do NOT break the cross-pieces, or you'll have to replace the drain itself.)
Remove all of the old putty. Roll a roll of fresh plumber's putty in your hands about the size of pencil, and wrap it around the drain hole in the depression under the lip of the drain, and snug the drain back down.
Let us know a little more information, and we'll be glad to help you.