Plugin Manager

It's not possible to access all the plugin's you have, nor that you've downloaded. But you can view the plugin's you've downloaded on your system.

Windows: Open up studio and goto Tools > Open Plugins Folder. Once that's opened up, at the top of your file explorer you should see a little part called "Roblox". Click that. Once you're there, you should see a folder called "InstalledPlugins". In there is where all the plugins you've downloaded off the site are stored. The name's of the plugin folders, though, match the id of the plugin on the site. Inside each is a .rbxm file, a roblox model. To insert this, you need to open a place (it can be a new one, doesn't matter) and go to Insert > Model and navigate to that rbxm. If you insert it, you should get what the plugin author submitted to roblox as the plugin.

Mac: If a mac user could contact me on steps showing how they would do this, I'd appreciate it. As far as I would think is that there is also a "InstalledPlugins" folder inside of the roblox directory, so try following the steps from the windows part. I was told it was something like Documents > Roblox > InstalledPlugins.