Did you not get software from Topaz along with the signature pad? You should have gotten a file called signplus.exe (or gemsignplus) that installs the software that integrates the Topaz signature pad with Acrobat. Then you need to go to the Signature Creation preference and select Topaz as the default signing method. You can also select Always ask and then you would be prompted to select either Topaz or the traditional Acrobat digital signature when you click on a signature field.
How to get to the signature preference depends on the version of Acrobat you are using. If you let me know I'll give you the exact steps.