Here are the instructions to use MSB repository through slackpkg+

1. Download and install slackpkg+ package from http://slakfinder.org/slackpkg+/
2. Add msb on REPOPLUS line so that it will become like this
   REPOPLUS=( slackpkgplus alienbob restricted slacky msb )
   You may remove other repositories (except slackpkgplus) if you don't use it 
3. Add msb repository information and match it with your Slackware and 
   MSB version, and your architecture
   Example:

   For Slackware 15.0 (32 bit) and MATE 1.26:
   MIRRORPLUS['msb']=http://slackware.uk/msb/15.0/latest/x86/

   For Slackware 15.0 (64 bit) and MATE 1.26:
   MIRRORPLUS['msb']=http://slackware.uk/msb/15.0/latest/x86_64/
4. Run slackpkg update gpg && slackpkg update
5. You can start managing your MSB packages through slackpkg+