Error using commands from vim

[Wallace, Brooke]

I'm getting the following when trying to use global from vim:

Error detected while processing function <SNR>8_RunGlobal..<SNR>8_ExecLoad:
line   57:
E492: Not an editor command:     cexpr! result

I'm completely new to this and am just following examples in the documentation. 
Steps I followed were:

gtags
htags
mkdir -p ~/.vim/plugin
cp /usr/local/share/gtags/gtags.vim ~/.vim/plugin
vi main.c
:Gtags -r SomeFunctionName

Can anyone tell me whats going on?

-Brooke