Pro Tools Express not compatible with Mavericks or Yosemite
 

See Pro Tools Express 10.3.4 Info & Downloads for info including the supported Operating Systems.

Pro Tools 11.0.3 adds Mavericks support, Pro Tools 11.3 adds Yosemite support.

Owners of Pro Tools Express bundled with Mbox/Fast Track interfaces can upgrade to Pro Tools 11 here, and this upgrade was recently reduced from $499 to $199.