You will need to know the Specific Activity of your enzyme in Units/mg.  You do not state whether you need to know mg of pure enzyme or mg of a given preparation - be sure you know the difference!  

Standard International Units measure how much active enzyme is present, and are usually defined as the amount of enzyme catalyzing one micromole of substrate consumed or product produced per minute under optimal conditions. This can change in an enzyme preparation over time, as the enzymes can lose activity as they age. The reason we use units instead of mg is for that reason.  Your preparation may have the same mass of protein today and next month, but the activity (Units) can change. 

Once you know the specific activity of your enzyme preparation, then you can calculate the mg of enzyme present.

I agree with Greg answers ...good luck .

Thanks for the explanation! Now I know what to look for. Fingers crossed, I hope the specific activity is easy to find. Great ;-)

you're right Greg! Thanks