Images in Mac Photo are in a specific folder, and I believe when you do a file/open command in Elements, you can navigate to that folder and load images. Once the image is in elements, create a new, duplicate layer. Here is a very simplistic guide:

http://www.dummies.com/photography/photo-software/adobe-photoshop-elements/how-to-work-with-layers-in-photoshop-elements-11/

Once the layer loads, assuming Topaz filters are already installed, access them with the Filter menu. The Topaz filter window opens, you do your thing then select the Save command, which will get you back to elements.

Here's a Topaz quick guide:

https://support.topazlabs.com/hc/en-us/articles/216538448-Accessing-Topaz-Labs-in-Photoshop-Elements

Once back in Elements, play around with the Layer Opacity command, which lets you dial in how much of the Topaz layer is blended with the original image.

Have fun.