The same way you did on the MacBook.
1. Install Lightroom.
2. Start Lightroom.
3. When prompted, enter the serial number you already have.
4. Start using Lightroom on the desktop the same way you did on the MacBook.
If you have the creative cloud version, simply launch or install CC desktop on your computer, go to the Apps tab and click the install button. If also using Photoshop, it's best to install Photoshop CC 2014 first, followed by lightroom.