johnski,
What is included in the .ZIP file? I downloaded a style from SketchUp Artists and the only thing in the .ZIP file is a .STYLE file. What do you see in yours? You say a .PNG?
On a windws OS the default styles are in the following folder: C:\Program Files\Google\Google SketchUp 8\Styles. I am assuming that on a MAC the directory is /Library/Application Support/Google SketchUp 8/SketchUp/Styles.
Once you get your hands on the .style file you can simply add it to one of these folders or load it directly into SketchUp from whatever location it was saved to. Reference1 explains this process.
Pro is not required to load a style into SketchUp. You would only need Pro if you wanted to create your own style. This is done through Style Builder, a separate application that comes with Pro.
TBS
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