To access Open Quickly, click the loupe located at the far left of the path bar in any editor tab, or use the keyboard shortcut ^Q (control-Q). Type a few letters of the filename or file content you're looking for and choose a file from the list of matching results.
Note: Open Quickly uses your Mac's Spotlight feature to locate files, and may not work as desired if Spotlight is turned off.
You can limit your file search to a particular folder by choosing Choose Folder… from the action menu.