AssetExplorer - Missing fields on Import / Custom Mapping

AssetExplorer - Missing fields on Import / Custom Mapping

Hi, I'm trying to import CIs/Assets for our new IP Phones, but when I go to do the mapping it doesn't seem to list all the fields.  Specifically I'm needing to map the MAC Address to each device (I have this information in my CSV file), but it won't let me assign it to the Mac Address field in AssetExplorer.  I can see that the CI type I'm importing to *does* have this field, but for some reason I can't import to it?

Anyone seen this before?  How can I get that Mac Address data into the system?
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