I ussed to have that problem until I used this advice from Stephanie Griffin of Topaz Labs. [This assumes you are using Windows and not Apple]
Please go to your preferences menu in Photoshop and select Plug-ins. Check / enable the Additional Plug-ins Folder option then click Choose. After that, please navigate to: C: \ Program Files* and select the Topaz Labs folder. Then click Choose (or it may say Save / OK) to save this back to Photoshop. Back in Photoshop click OK to save your changes, then restart Photoshop. Once Photoshop opens, go to your Filter menu and you should be able to access Topaz.
*on a 32-bit system or c:\program files (x86) on a 64-bit system
BTW, you must restart Photoshop after making these changes.