The correct name of the file is user_dlts, not dlt_users. I'm not sure if that's what you meant to write? This file is applicable per-profile though, so if you're running Wireshark using a different profile other than the Default profile, you'll have to update the per-profile file as well.
You could also launch Wireshark using the -o "uat:user_dlts:..." option, which should then work regardless of the profile. The Wireshark man page provides the following example:
-o "uat:user_dlts:\"User 0 (DLT=147)\",\"cops\",\"0\",\"\",\"0\",\"\""