. Locate the file "Adobe Photoshop CS5 Prefs.psp" on your computer and delete it or move it elsewhere. (I assume by doing this you may lose * of your photoshop preferences but it will allow you to use photomerge and HDR so you choose)
2. On Windows this file can be found in the User folder under C:*USER NAME*AppDataRoamingAdobeAdobe Photoshop CS5Adobe Photoshop CS5 Settings (Replace *USER NAME* with the Windows 7 account user name)