Try this..in Cydia, go into "Changes" and try and hit the Refresh button in the upper left.. I had this happen where I didn't have the latest packages installed.
I just did a search for N6 HD and it came up with a message stating that it is not known if the theme (product) was supported by my iOS version.. I'd suggest sending a message to the author to see if this theme is compatible with iOS5.x.x