Presumably, you don't have any of the following programs on your machine, so the
variable is set to nil:
(defcustom org-mobile-checksum-binary (or (executable-find "shasum")
(executable-find "sha1sum")
(executable-find "md5sum")
(executable-find "md5"))
"Executable used for computing checksums of agenda files."
:group 'org-mobile
:type 'string)
If you have some checksumming program, customize the variable
to be the pathname of the program. If not, you'll have to get one.
This was discussed on the list before:
http://thread.gmane.org/gmane.emacs.orgmode/19702/focus=23857
HTH,
Nick