There is a new way to install ROBLOX Studio Plugins. The old way still exists, but you can now install ROBLOX Studio Plugins from the official ROBLOX website!
This is the ROBLOX Blog article by the ROBLOX Staff themselves about it:
http://blog.roblox.com/2014/01/using-plugins-just-got-10x-easier/
Basically, you can upload plugins just like models, and install them just like models. You select the plugin from the link below, press Install, and that’s it!
Here is the link to install ROBLOX Plugins uploaded to the ROBLOX website:
There are already a ton of Plugins to install! Try them out!
I will upload the Plugins that I am working on to ROBLOX, and provide those soon in the ‘Plugins’ section of robloxbuildingguide.com! Don’t worry; all previous plugins installed from other websites should still work perfectly!
Have fun installing ROBLOX Plugins
— cardgamechampion, Author of “ROBLOX Building Guide”.