Object or Field Not Visible From dataloader.io

Solution

If you are unable to find a certain object or a field of an object in the dataloader.io UI, you'll need to verify that you have appropriate permissions over the offending object.

As a rule of thumb, any users that need to import data into a specific object/field, need to have the "write" access enabled for that entity while exporting needs "read" access.

Unfortunately, if the cause is that object/field was introduced in Salesforce v38 or later then dataloader.io from MuleSoft won't support it for the time being.

You can validate in the Salesforce SOAP API Developer Guide if the object/field is listed for v37. In case you are not able to find it in v37 (and you can in newer versions) then that object is not supported.