When I tried the dpkg -i merhod, having installed json in red pill mode, I got

Package python-simplejson is not installed.
dpkg: error processing mitter (--install):
dependency problems - leaving unconfigured
Errors were encountered while processing:

which is basically the same as I got when trying to use app manager. But when I used qwerty's link to install simpljson, app manager worked.

So Mitter works fine now, thanks.