Greets Hrothbert,
No problem mate, this one's a bit tricky. I'll try to go it through step-by-step. Note that this gui contains XML changes, and it will mess up the other skins! So be sure to back up your PS/data/gui before copying over any files.
1) Save the zip from above (do not unzip) into your PS/art/skins/ directory.
2) The zip contains XML files that need to be unzipped into PS/data/gui/, we'll need to get those out from the zip
3) If you don't know how to do the above, duplicate the whole zip.
4) Unzip the whole duplicated zip into a temporary directory.
5) Go to that directory, and you should see multiple subdirs, and two files.
6) Delete everything else on that directory, except the subdirectory "data/".
7) Go into your PS/data/ directory and take a backup copy of the entire gui/ subdirectory.
8) Go back into the temp directory and move the data/ directory into your PS/ directory.
9) Ready! Run PS setup and select GP_radical skin.
I think the auto-unzip is a Mac feature. To circle around it try this:
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If you try downloading it via Safari, it may decompress the zip file (single left click opens a new window and then starts downloading). This is what happened to me the first time and rezipping the downloaded folder didn't help.
Instead, try right clicking (or for those poor souls with a single button mouse then Control and click) and choose Download Linked File - this doesn't open a new window but instead just starts the download process. Do not unzip the file. Follow the rest of the instructions as stated and it should load up.
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Thanks for your interest,
- Cherppow