Hello, Nate.
Could you please clarify, what are you trying to achieve?
If you want to use NDI as input, please select NDI as a source on the top of the 'Edit source stream' dialog, it will be decoded. No decoder will be available in this case.
If you want to convert NDI to another format, then choose codec to encode to. If it is not working, probably the NDI stream is not received by Nimble.
Please execute nimble_ndi on the Nimble server to check for available NDI streams for input.
I also suggest launching a ticket at wmspanel.com/help to continue the discussion, as we can request some sensitive information.
Thank you.