You are trying to open the 'WenEntry_Form.htm' and this is not a compatiple OfficePool file. This is an HTML file and can't be open from within OfficePool (This was created by OfficePool but it's just a web page). You need to open the file that ends with the "opl" extension.
NOTE: OfficePool opens the last file you had opened on launch. So if OfficePool starts with this error you will need to open a legitimate OfficePool file
and then close the app and restart. OfficePool should now open the legitimate
file upon launch.
If not, you can use the "Reports->Show Templates Folder" menu item and it will open a folder which contains the file "com.pachysoft.officepoolmacapp.prefs". Rename or remove this file and restart OfficePool. You will have to re-enter your registration information and this will also reset the file you last opened.