You don't. Express Scribe can't play DS2 files. The Olympus DSS Player Pro can be set to record on Clasic instead of Pro to generate DSS files instead. Here's a link describing how to do it.
http://support.recorders.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=131
There are also converters that will convert from DS2 to WAV for files already recorded. Try here for that.
http://www.filecluster.com/Audio-Video/Converters-Rippers/Download-DSS-File-Converter.html
Good luck!