Cannot activate via an add-ins window in XL 2007

I can see the Addin (MathNET.Division) in the InputBox.

I can select/deselect it.

That said, I get a #NAME? when I enter into a w/s cell the formula "=DivideBy4(1234)"

DivideBy4() is a class function in MathNET.DLL specifically MathNET.Division

The problem is but that this add-ins is in the Inactive Application
Add-ins section and I can't change it to the Active Application
Add-ins!? I have tried with checking them after choosing Manage and Go, but
nothing happened - also no failure message either.

What can I do in such situation? What can I do to get this Microsoft add-ins active?

TIA EagleOne