George,
Go bach into SMC and bring up the properties for your library. On the Options tab, press the "Advanced Options..." button. The first tab, Pre-Assign, has one checkbox, "Library is pre-assigned". By default, library is pre-assigned is unchecked. For your needs, you will want to enable this checkbox turning on pre-assignment for this library.
Pre-assigning libraries defined in metadata will force the underlying engine, in your case BASE, to be used instead of the META engine which renders the data read-only in the Add-In.
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