Contact(ENG,GER): unrealmarketplace@virtualbird.de
Plugin is located, after the installation via the launcher, in the directory: Engine\Plugins\Marketplace
If you use the Unreal GIT version you have to copy the plugin into your project: D:\myproject\Plugins\WebCommunication
On OSX or Linux or if you are satisfied with the plugin and don't want any updates you can also copy the plugin into your project.
The plugin should be activated automatically after installation. If not, you can activate it via the plugin menu.