Try to do something like this.
using (SqlConnection connection = new SqlConnection("Your connection string"))
{
connection.Open();
using (SqlCommand command = new SqlCommand())
{
command.CommandText = string.Format("ALTER TABLE TABLE_NAME ADD {0} {1}", "[txtColumnName.Text]", "[cmbColumnType.SelectedValue]");
command.Connection = connection;
command.ExecuteNonQuery();
}
}