Divya,
This functionality is supported in TDM 4.3 which ships with FDM 4.7. I see the IMaskFunction interface in the 4.7.200.9 jar file. TDM 4.2 ships with FDM 4.7.0.14 which does not come with this interface.
Please upgrade your FDM to at least the version shipped with TDM 4.3 (though TDM 4.5 is currently the latest GA version) and use the jar file from that version.
See the following document for details on downloading products from our support site:
How to download your product from CA support portal
Hope this helps,
Sean