Topaz ReMask 4 requires a host program that offers layers and transparency in order to function, which are not supported in Lightroom, Irfanview & PhotoImpact.
In order to use ReMask you will need a program such as Photoshop, Photoshop Elements, PaintShop Pro or Serif PhotoPlus. You can also use our stand-alone program photoFXlab as well. You do have the option of using photoFXlab as an external editor through Lightroom and accessing ReMask in that manner.
For configuring ReMask 5 in Lightroom, please see the article below: