I'm writing down how I managed to get xmplay skins to load onto the portableapps version 3.7 of xmplay (in case anyone else has the same trouble).
XMPlay's support site says you can place the skins (.xmpskin files) "anywhere". See their article "Installing Skins, Plugins and Visuals":
http://support.xmplay.com/article.php?id=4
First, I made a "Skins" folder (since there wasn't one in my xmplay portableapp) and placed all my downloaded ".xmpskin" files in it.
But I found that placing it just anywhere in the xmplay portableapp version didn't work. None of the skins got loaded no matter where I seemed to place it. I would move the folder around, restart xmplay, and check by right-clicking on the title bar to bring up the skin options; as well as by going into the Options&Stuff>Appearance menu. Nothing.
By trial and error, I finally got the skins to load by placing my new "Skins" folder here (this is what the file's address looks like):
(my portable drive)/PortableApps/XMPlayPortable/App/XMPlay/Skins
This is what it looks like when you explore the PortableApps folder in Windows:
PortableApps
____XMPlayPortable
_______App
__________AppInfo
__________DefaultData
__________XMPlay
_____________Skins
And now I have my fancy new skin on! Hope this helps other xmplay users. 
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This. Thanks, miktab, this is what I was looking for.
In future releases, it would be nice to already have the Skins folder present, even it is is empty, to guide people as to where to save the skins.
Either this or make a Skins folder in the Data directory. That way you can copy the files before running the program and restoring them when the program is closed.
The Data folder is where users will most likely look, since they know how for PA Format works.
A more optimal solution would be if it's possible to set the folder where the skins are stored in the program or using an ini file. That way we can just set it to point to Data\Skins as default. This however isn't possible in the current release.
Yes, I set the working directory!