Files are uploaded as a part of creating a message (see the help section on creating messages). After the message itself has been entered, the Upload command is used to upload the file to be attached.
Upload uses XModem to transfer the file. All Macintosh files should be transfered using MacBinary as well. Therefore, be sure to set your file transfer program to XModem/MacBinary mode.
In addition, larger files or groups of files should be packed using PackIt III before being uploaded. This program is available in the General message section (try Filtering on "Files: yes" and "Keywords: PackIt").