When using the test function in Facebook Ad Manager you have to click the 'get' or 'install' button to activate the pixel. If the pixel isn't found after that, you can use the Facebook Pixel helper (chrome extension) to test it. 

To test pixel activity with the chrome extension, open a preview of one of the Geeklab variations and click on the 'get' or 'install' button of the look-alike page. Because the pixel will only be active for a short period of time, it is useful to pin the Facebook Pixel helper to your browser bar so that you can quickly click on it.