This example creates a field named TestField in an existing table, Table1:
Dim db As DAO.Database
Dim tdf As DAO.TableDef
Set db = CurrentDb()
Set tdf = db.TableDefs("Table1")
tdf.Fields.Append tdf.CreateField("TestField", dbText, 80)
If you prefer, you can do it by executing a query statement:
Dim db As DAO.Database
Dim strSql As String
Set db = CurrentDb()
strSql = "ALTER TABLE Table1 ADD COLUMN TestField TEXT (80);"
db.Execute strSql, dbFailOnError