I bought from StartSSL 'Class 2 validation'. Then I successfully created the Object Code Signing Certificate (PFX) and successfully imported my certificate into Visual Studio 2010.
Imported certificate in VS 2010 http://dl.dropbox.com/u/80507/signing/configuration.PNG
When I try to sign the VSTO Manifest (for PowerPoint Add-In ClickOnce setup), I get this error:
An error occured while signing: The parameter is incorrect.
Signing error http://dl.dropbox.com/u/80507/signing/error.PNG
I spoke with StartSSL and their certificate would not have a problem with this.
If you do not specify StartSSL timestamp server, the certificate (and ClickOnce manifest signing) works without problems.