Check if /usr/local/lib is in your /etc/ld.so.config file. If not, add it and run the "ldconfig" program as root.
/usr/local/ is where software is supposed to go that you install yourself. Details are in the Filesystem Hierarchy Standard, which is available at the www.tldp.org
The /usr hierarchy is where a slackware .tgz package installation will go. If you install a non slackware tarball it may go under /usr/local.
You can do as MMYoung suggests when installing packages in the future. However, if you already have software installed in /usr/local, make sure the PATH variable has /usr/local/bin in it, and that the /etc/ld.so.conf has the /usr/local/lib/ entry. The only thing you would lose by using /usr/local is possibly losing this software when you do an upgrade, or go with a different distro. A distro will not touch /usr/local unless you format the partition it is on. This makes /usr/local a good candidate for its own partition. More so for a server environment.