First find out where your KDE is installed:
bash-2.05a$ which konsole
opt/kde-3.1/bin/konsole (mine is under /opt/kde-3.1, yours is probably somewhere else)
Then ./configure --prefix=<path-to-kde> (in my case it would be --prefix=/opt/kde-3.1)
If configure doesn't report any errors you can do make.
To install your theme system wide you have to become root:
bash-2.05a$ su
Password:<your-root-pwd> (mine is... heh, I am not going to tell you all my root pwd
)
Now you can do make install, and voila! Plastik should be right there in the control center.
edit:
Don't forget to exit from the root account after doing make install or you might brake up your system really bad...
bash-2.05a# exit
exit
bash-2.05a$