Hey Elijah,
Your DEM is in a Raster format. Typically this means that each pixel in the raster has a corresponding elevation. I have not had much success myself working with these sort of files directly in TBC. You would need to convert them into a point file or some other format in ARC GIS, or FME or something to be usable in TBC. If the DEM was derived from LiDAR, your best bet might be to find the raw LiDAR data that the model was created from as TBC is great with .LAS and .LAZ files.
Sorry I am not much help here, just thought I'd share my experience.
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Scott Lehr
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