Cool
My first thought is - it's complicated!
What you will want to do is modify the datafields for an activity (it's a datafield so only gets displayed when you have an activity running). E.g. you might already have (or might want to make) an EMTB activity - and add it to that. (Or just try adding it to any activity).
Note that even when you are customizing the datafields for an activity when you add the "STEPS EMTB" field then it should try connecting to your bike.
This is the manual page which might be the best explanation:
vívoactive 3 Music - Customizing the Data Screens
The "STEPS EMTB" field will be listed under "ConnectIQ Fields"