There is several ways:
1. You can find "MyColumn" in the FlyGrid.Columns
flyGrid.Columns.GetColumnFromFieldName(string fieldName)
and use Column.SetValue(NodeBase node) method to fill the cell of the node.
2. You can find the column:
int cellIndex = flyGrid.Columns.GetColumnIndexFromFieldName("myColumn");
and use NodeBase.SetCellValue:
someNode.SetCellValue(cellIndex, 100);
to find Node in the Rows you can use FlyGrid.Rows.GetNodeFromRow:
NodeBase someNode = flyGrid.Rows.GetNodeFromRow(0);
If you using table structure (without hierarchy) you can access to the node from row = 0 directly:
NodeBase someNode = flyGrid.Rows.Items[0];
3. If you want to fill whole row of the node you can use Node.Value property.