See here for usage example.
Changing look so its not a button
DataGridViewComboBoxColumn ^DataGridViewComboBoxColumn1 = gcnew DataGridViewComboBoxColumn(); DataGridViewComboBoxColumn1->FlatStyle = FlatStyle::Flat;
You read the text contents rather than the selected index. Don't use ->ToString() as it causes errors with nulls.
if (Convert::ToString(dataGridView1->Rows->Cells->Value) == "")
Feel free to comment if you can add help to this page or point out issues and solutions you have found. I do not provide support on this site, if you need help with a problem head over to stack overflow.