You need to specify the datatype (dbText) and field size e.g. 50
Code:
Set newField = tdf.CreateField(newFieldNameVar, dbText,50)
I've not tried it with a variable rather than a field name in quotes, but I can't see why it wouldn't work
See this link on Allen Browne's website for many more DAO functions related to setting or modifying tables/fields/indexes/relationships using code http://allenbrowne.com/func-DAO.html
@June7
The code also works on linked tables which is very useful for updating backends in split databases
I use this approach to modify backend databases when distributing frontend updates to clients without needing to visit their site
e.g. add new tables, add/modify fields, add new indexes etc