@JanGoverde, I suppose the tools that you are using are 'script tools' created in toolbox. If it is a script tool, you can follow the process to set the data type, so the drop-down appears:
Open the Properties of script tool.
Open 'Parameters' tab, set the parameter (data type) of 'Map Unit Layer' as 'Feature Layer'. (Assuming Map unit layer is a feature layer)..
Save the changes & refresh the toolbox.
If layers are added to the map. The drop-down will be displayed.
@JanGoverde Let me know if this doesn't work & it would be great if you could share the SS of toolbox & tool properties.