Some files created on Apple operating systems (Mac OS X) are stored as packages, which is a folder that Mac OS X interprets as a single file.
To upload package files, you will need to use Box Sync 4.0+ or convert them to be a saved as a single file. See Apple's support documentation for more information on how to convert iWork package files to single files.
Other applications may also offer the option to save these types of files as flat files, which can uploaded without using Box Sync.
Alternatively, you can right-click the file and compress it (save it as a .zip file) before uploading to Box.
See Mac Package Support for more information on uploading package files using Box Sync.