Unable to trust makecert signed COM Add-in (Outlook)

I have a COM Add-in made for Outlook that I have signed with a test certificate made with the makecert.exe utility. When starting Outlook, the add-in loads and a security warning comes up. The problem is that the checkbox that allows me to 'Always trust macros from this publisher' is disabled.

Is this because I have not used the right options with the makecert.exe utility or do I need a proper certificate to be able to trust the add-in in Outlook?

The following options were used:
makecert -sv SomeKeyName.pvk SomeCertName.cer -n "CN=Some common name

And then:
cert2spc SomeCertName.cer SomeCertName.spc

Finally:
signtool signwizard (using the files created)

Any help appreciated.

[741 byte] By [Foss] at [2007-12-18]