To set the value of a CheckBoxCell you need to cast it specifically like this:
(dataRow.Cells["your cell index"] as DataGridViewCheckBoxCell).Value = true;
This presumes dataRow is your current datagridview row. Hope this helps
...and I have extensive experience writing computer code, including OIC, BTW, BRB, IMHO, LMAO, ROFL, TTYL.....