It might be because berkshelf is not installed with other Vagrant gems, which is my case since I use RVM to manage multiple ruby environments. The stacktrace looks like this:
% RIAK_CS_CREATE_ADMIN_USER=1 vagrant up
/Applications/Vagrant/embedded/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- berkshelf/vagrant (LoadError)
from /Applications/Vagrant/embedded/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
from /Users/jbbarth/dev/riak-cs/vagrant-riak-cs-cluster/Vagrantfile:7:in `<top (required)>'
[...]
I'm not sure I have a precise idea on how to fix that in a generic manner, but fortunately it can