I only find information on importing .d5a files using the ‘import’ button, but it doesn’t work for .d5m files
Currently, D5 Render cannot import .d5m files directly using the Import button. These files are usually material or asset packs and should be added via the Assets Library instead.
If the .d5m file contains models, you’ll need to convert them to a supported format, such as .d5a or .fbx. Let us know what type of content your file has, and we can guide you on how to use it in D5 Render.
You may also check out the details here: Import | User Manual