If you don't see any "plugins" listed in the "plugins" page of the configuration dialog after installing Kopete you may have installed it in a directory where KDE doesn't look for additional resources like plugins.
KDE applications by default look in all directories listed in the /etc/kderc file, the $KDEDIRS environment variable, in $KDEDIR if this variable doesn't exist, and ultimately in the directory where kdelibs is installed. Unless you take special precautions applications will not look in other directories, even if you install additional software (like Kopete) there.
The best solution is to add the following to /etc/kderc (create the file if it doesn't exist yet):
If you don't have write access to /etc/kderc, or if you want to use more parallel builds of KDE and/or Kopete you need to rely on the environment variables instead.
If $KDEDIRS currently points to /opt/kde3 and you installed Kopete to /usr/local then you need to set $KDEDIRS to '/opt/kde3:/usr/local'. You need to store this change in a file that is picked up by all shells and for all users, like /etc/profile. The name of the preferred version varies from system to system. After changing /etc/kderc or $KDEDIRS you'll have to update your system settings by running kbuildsycoca.
*Remember, Kopete only works with KDE 3.4.0 or above!
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