slackware will install KDE to /opt/kde so look there. if there is kde installed--which i doubt--type ($ = user prompt, you DON'T need to be root. that's a dangerouse and bad habit to do everything with sudo/root permissions!)
if your system tells you 'command not found' and you are absolutely sure kde is installed than you have to ajust the PATH variable. the PATH variable tells the system where to look for executables ("commands"). if kde is installed e.g. in /opt/kde you should check
it will print out a somewhat cryptic line like PATH="/bin:/usr/bin:/usr/local/bin" and so on... the ':' seperate the directories in which your system looks for executabels. you must have "/opt/kde/bin" in your PATH. if not do:
$ export PATH="$PATH:/opt/kde/bin"
if it still tells you "command not found" kde is either somewhere else (very unlikely) or you have it not installed (most likely).
to install kde just insert the cd-rom with kde on it in your drive. mount it an take a look in the slackware/kde folder. then do as root (# is root prompt):
and start at the beginning
good luck, vadkutya