I'm trying to install the Aqua OS K window decoration (from kde-look.com). I'm installing it via the Mandrake RPM version, but when I try to install it, it tells me that it requires libGLcore.so.1

I have libGLcore.so.1 in /usr/lib

So why doesn't it work?

Check 2 things:

Are you root?

Open up a console and type:
less /etc/ld.so.conf

Is /usr/lib listed? If so, then click:
q (this quits less)
And then type:
ldconfig (make sure you are root)
While you are at it, type:
updatedb
To make then both run while you go get your new cup of Joe ready, type this instead:
ldconfig &&
updatedb

Then after 10 minutes, when they finish, try installing again.

Cool

Hmm, actually /usr/lib is not listed. Should I add it?

...............
(edit) I added /usr/lib to the file, and then ran ldconfig && updatedb

I tried installing it again, but got the same error message. Any ideas?

AFAIK, /lib and /usr/lib don't go into /etc/ld.so.conf [...not needed as those paths are hardcoded into the linker itself ...is my guess]
In fact I tried that in the past and it introduced new bugs [...YMMV], so if it didn't work anyway, you may be better off taking it out. [...if you do, run #ldconfig again afterwards]

Use the rpm tools to first see what pkg supplies libGLcore,
$urpmf libGLcore.so.1
...see if the right pkg is already installed,
#rpm -qa | grep <pkg>
...and install, if required,
#urpmi <pkg>

If rpm -qa shows the <pkg> as already installed, it could mean you don't have the right version installed to satisfy the new app's dependency.

Go to this URL to download the libGLcore.so.1.rpm:

http://lists.uugrn.org/uugrn/02/12/7971.html

This file is un driver for the Nvidia cards.

Install it and reinstall the AquaOS, that's all...