If you click Start Lab and a loading window does not pop up, it is almost always due to a pop-up blocker.
See https://drive.google.com/open?id=0B-6sMEg5M-7VRU1VdTZ5LTg2VjQ for a video guide of how to enable our pop-ups or follow the picture tutorial below.
1. Click Start Lab.
2. Click on the Pop-up blocked icon in the top right area of the address bar.
3. Click Always allow pop-ups from http://cdn.testout.com then click Done.
4. Click Stop waiting for the simulation window to open.
5. Click Start lab again.
The simulation window will then open and begin to load the lab. If it doesn't, then completely close down Chrome and repeat the above process.
For other browsers, see https://testout.zendesk.com/hc/en-us/articles/229941168