The short answer is that if you install the DSS Player application that comes with their digital voice recorders, and then install the 'DSS Player for Mac (OS X) V6 Update to V6.0.4' — which can be found using their Support - Downloads Web page — then the DSS Player software will work with Tiger.4. Since your Mac Mini is PPC, your only real option for running Windows is Virtual PC. There are some other, open-source projects for getting Windows to run under Mac OS X, but Virtual PC is probably the 'gold standard' in that arena.
They are planning to update their Web site to reflect this as well as provide a FAQ. The basic instructions are the following:
• If your Olympus digital voice recorder comes with DSS Player V6, you simply install 'DSS Player for Mac (OS X) V6 Update to V6.0.4' using the instructions in the Read Me file.
• If your Olympus digital voice recored comes with DSS Player versiion 1.xx, you first install the 'DSS Player for Mac (OS X) Upgrade from version 1.xx to V6' using the instructions in the Read Me file, then install 'DSS Player for Mac (OS X) V6 Update to V6.0.4,' again following the instructions in its Read Me file.
StuffIt Expander is required to unpack these downloads. Tiger, unlike prior versions of Mac OS X, does not include Stuffit Expander: see 'Mac OS X 10.4: Where is StuffIt Expander?' If you don't have StuffIt Expander, a free download of such for Mac OS X 10.4 can be found here.