Adding more maps?

[blooalex18]

Go to servercmdline.txt in /config and scroll down to where it says:

[SELECT ALL] Code:

//-M     // sendmap permission string

Take off the first two /'s in front of -M.
Now put cUR after -M so it looks like:

[SELECT ALL] Code:

-McUR     // sendmap permission string

Now anybody can upload maps and admins can update and revert maps.

I tried doing that but I get this error message every time I try saving it. The message is "The document “servercmdline.txt” could not be saved. The volume is read only." Is there some sort of other way to fix this? Btw I use a mac and used text edit.