This is SqlDataAdapter that I use for filling dataGridView1 but before I fill it I add DataGridViewComboBoxColumn to that datagridview So
Your DataGridViewColumn must be added to DataGridView before you set .DataSource.
Then set .DataPropertyName for new column:
cbColumn.DataPropertyName = "PLODINA"
DataPropertyName must be name of the field from "PRONAJEM" table.
And then item of comboboxcolumn where .ValueMember = PRONAJEM.PLODINA(from DataTable) will be selected