The HMM output files are in a compact binary format. Though I've taken care to deal with byte-swapping issues between architectures, I can't guarantee that this format is universally portable. To make HMMs which can be read in a machine-independent manner, use hmm-convert to convert the file to a flat text (ASCII) format.
Flat text models are also useful for emailing to people, or for modifying yourself (say, with Perl scripts). Their format is (excessively?) verbose and should be self-explanatory.