If you're running Slackware 14.1 (or pretty much any other up-to-date distribution) as a VirtualBox guest, you need to create a symlink in order to get 3D acceleration working.
mkdir -p /usr/lib/dri
ln -s /usr/lib/xorg/modules/dri/swrast_dri.so
(I assume that if you were using 64-bit slackware, you would put the symlink in /usr/lib64 and not /usr/lib).
This workaround was described on their forum way back in 2009:
You could test it with glxinfo (the vendor string should say Chromium), but it's more fun to test it with Steam.