One thing you can can check is that as indicated this documentation: http://www.salesforce.com/us/developer/docs/api/Content/sforce_api_guidelines_personaccounts.htm
"Salesforce provides one default person account record type, PersonAccount, but an administrator can create additional person account record types."
And our Donations tool assumes that this is the case.
The problem may be that the name of that record type has been changed in your account. Instead of "PersonAccount" it's "Person_Account" or "Person Account" or something like that.
If you look at the "Record Type Name" in the Person Accounts record type, you may see that.
Return the record types name to "PersonAccount" and this issue should be resolved.