yea what rotvogel said.
rpms dont really work in slackware .... slackware uses tgz files but you will only find them on slackware mirrors and
www.linuxpackages.net .... typically you have to compile from source files in slackware.
but if you want to install a rpm do
cd /path/you/want/to/create/the/tgz/file/in ie. cd /home/$USER
then:
rpm2tgz /path/to/rpm/crossover-x.x.x.rpm
this will create crossover-x.x.x.tgz in the directory you are in so now type
su
installpkg ./crossover-x.x.x.tgz
***HOWEVER***
what is the file you are trying to install crossover from i think crossover installs from a sh script like mozilla does. just run the install-crossover-standard-demo-3.0.1.sh file ... this is the demo file but check your crossover cd or what you downloaded for something like that.