Your code seams correct.
If you have a datatable with one column like this:
Test
====
123
456
And apply your code, you will get the string "123, 456"
// Init datatable
var dt = new DataTable();
dt.Columns.Add("Test");
dt.Rows.Add(dt.NewRow()["Test"] = "123");
dt.Rows.Add(dt.NewRow()["Test"] = "456");
// Join columns
string s = string.Join(", ", dt.Rows.OfType<DataRow>().Select(r => r[0].ToString()));