The useFullscreenExpand configuration option is supported by both Juicebox-Lite (the free version) and Juicebox-Pro.
Here is a sample Lite gallery which sets useFullscreenExpand="TRUE" (and the gallery expands fullscreen rather than just full browser) when clicking the Expand Button on the Button Bar.
www.juicebox.net/demos/lite/full/?usefullscreenExpand=TRUE
The Fullscreen API is supported by all major modern browsers.
Please see here for a list of browsers which support this feature: http://caniuse.com/#feat=fullscreen
Please check that your gallery's 'config.xml' file contains useFullscreenExpand="TRUE" as an attribute to the opening <juiceboxgallery> tag. You can open the 'config.xml' file in a plain text editor to check its contents.
As long as you select the 'Use Fullscreen Expand' checkbox in JuiceboxBuilder-Lite, the option should be included in the XML file.
If you have recently made changes to an existing gallery, then also make sure that the original XML file is being overwritten on your web server. If you are in any doubt as to whether or not the original file is being overwritten, try deleting it from your web server first before uploading the new version in its place.
Finally, make sure that you clear your browser's cache before reloading your gallery's web page to ensure that your browser is using the current files from your web server (and not older cached versions).
If you continue to experience difficulties, please post the URL to your gallery's web page so that I can take a look at the problem for myself and hopefully help further. Thank you.