If you have one outlet ("mySelection") and multiple rows, which row did you think the outlet connects to? (Answer: none of them. You probably hooked it up to the prototype cell, which is never displayed or used directly.)
But no matter, you don't need to check the state before you set it. Assuming your other code is correct, you should be able to do something like:
- (void)tableView:(NSTableView *)tableView setObjectValue:(id)object forTableColumn:(NSTableColumn *)tableColumn row:(NSInteger)row
{
MediaAComparar *medium = [mediasRecesEnStock objectAtIndex:row];
if ([tableColumn.identifier isEqualToString:@"seleccion"])
medium.seleccion = object.booleanValue;
}
Less code is better code.