If your Browser tab(s) in FL Studio 12 are empty there are two common causes, either your Browser snapshots are somehow messed up, or you've changed options in the Browser Options.
Browser Snapshots
To fix the Browser Snapshots download and extract the Browser.zip to "C:\Program Files (x86)\Image-Line\FL Studio 12\System\Config\Browser".
Browser Options
Open the Browser Options and make sure "Show only opened folders" is unselected.