LustyExplorer is not loading

I added lustyExplorer to ~/.vim/plugin. Then I ran MacVim and tried to run it with ":LustyFileSystemExplorer". Error

"Unknown Function: lustyExplorer#LustyFileSystemExplorerStart"

In the documentation it says:

Usage:
"                 <Leader>lf  - Opens the filesystem explorer.
"                 <Leader>lr  - Opens the filesystem explorer from the
"                               directory of the current file.
"                 <Leader>lb  - Opens the buffer explorer.
"                 <Leader>lg  - Opens the buffer grep, for searching through
"                               all loaded buffers
"
"               You can also use the commands:
"
"                 ":LustyFilesystemExplorer [optional-path]"
"                 ":LustyFilesystemExplorerFromHere"
"                 ":LustyBufferExplorer"
"                 ":LustyBufferGrep"

I also tried :lf and got Error:

can't open errorfile errors.err

Am I executing it correctly? What does the prefix <Leader> mean?