Starting with Version 3.xxx (not sure which) Topaz introduced a new "model" for Raw files and they made significant changes to the work flow. Since I usually update it when it prompts me to do so on opening, I did it without seeing without release notes, it passed under my radar (and that of other users if I can judge by the comments on their site).
To use the RAW model, you cannot use it as a plug-in. You must use DeNoise as a stand alone application, open the file outside LR (through show file/directory in Explorer). There is a Youtube video describing the workflow under the new release (the link is probably available on their site - sorry I did not save it). In a nutshell, after you open the file in Explorer and edit it in DeNoise, you must SAVE it AS a DNG file in the same directory, synchronize the directory and the settings in LR, stack the 2 photos and voilà. If it looks complicated, it's because it is.
You can also use DeNoise as a plug-in (here are the instruction to configure it:
DeNoise AI v3.3 Lightroom Plugin Update Instructions - Topaz Labs) . It work almost as before, with a major twist. By invoking the external editor, LR generates a TIFF file with all the settings and passes it to Denoise . Unfortunately Denoise as lost the ability to save the modified TIFF created LR and saves it under another name , forcing you to go through the same steps as above plus deleting the intermediate unedited TIFF file. (I looked, there is definitively not a "Save2 option, only a "Save as"). I spent several hours on their site trying to figure it out. I guess they favoured the RAW model (because you cannot save under the original RAW format and must save under a different format - DNG recommended, but strangely enough, it can save them but not read them) and forgot the need of the plug-in mode
My major problem is that RAW files don't open correctly in DeNoise. The colours shift, becoming magenta/pink. It is a known issue, but I cannot use it in that mode until it is fixed. The few times that I managed to open files correctly, The raw mode seem to a very good job.