I could not find any reason except the older DM only worked on 32 bit software and that may be one issue. Blackberry suggests upgrading the DM to support the 7100.
As for the 7100, there does not seem to be any reason it should not be seen unless there is a hardware issue.
this link is for 10.6. but may offer some help.
KB19728-BlackBerry Desktop Software is unable to detect the BlackBerry smartphone when the Mac computer is booted into Mac OS X 10.6 64-bit mode