The most common cause of problems with the DG-100 is lack of drivers. The DG-100 uses a Prolific USB Serial chip, and so the drivers from Prolific are your best bet.
If you aren't certain that you have the latest Prolific USB Serial adaptor drivers, go to the Prolific website and download the latest OS X drivers from the Support page (you may need to search for PL2303).
There are also now some third-party drivers. We have had some success with the failberg drivers and in particular the 0.4.1 version (the 0.5 version did not work for us in testing under Lion).