Step 1 - Drag & Drop
Open iTunes, select 'Apps' and scroll down; select iFileExpress on the left, drag & drop the file on the right.
You can now pin your favorite files to the top of the list by long pressing on the file and selecting "Pin to top" from the context menu. This makes it easy to get to your favorite files quickly even if you got a large number of files in a directory, no more heavy scrolling!
iFunBox for Windows: Open iFunBox, drag & drop the file to: User Applications > iFileExpress > Documents. You can either drop files or use 'Copy From PC' to the Documents folder, or a specific folder for certain type of files. For example, put PDF files in Documents/docs folder.
Same with iFunBox for Mac