1. Install the plugin either from plugin manager or reside "OpenAtlasTurkey" under the proper location depending your O.S. 
(e.g. for Windows reside it under C:\Users\%YOUR_USER_NAME\.qgis2\python\plugins\)

2. Once installed it will be listed under Web menu and also as an icon on plugins toolbar. 
(Do not forget to enable it from plugin manaeger)

3. Once you click on the plugin icon a list of avaliable service will be listed. Choose desired service and click "Add Layer" button.
(A list of avaliable services are stored in the "data_sources.csv" file that resides in the installation directory. You may edit based on your needs.)

4. Once you click on a service (dataset) and click add button, plugin first queries the service address to determine the # of layer provided in that address. 
(Plugin determines the # of layer in a service with the standart OGC GetCapabilities command.)

5. This is the initial release, I hope more services will be added soon.

Keep on using QGIS ;)
Best wishes.
Cem GULLUOGLU 
