How to select items in NSOutlineView without NSTreeController?

No, there isn't a selectItem: method, but there is an rowForItem: method. If you combine that with Peter's advice about using selectRowIndexes:byExtendingSelection: above, you should have all the information you need.

If you really wanted to have a method to select an item, which I would recommend calling setSelectedItem: for consistency's sake, you could write something like this in a category on NSOutlineView

- (void)setSelectedItem:(id)item {
    NSInteger itemIndex = [self rowForItem:item];
    if (itemIndex < 0) {
        // You need to decide what happens if the item doesn't exist
        return;
    }

    [self selectRowIndexes:[NSIndexSet indexSetWithIndex:itemIndex] byExtendingSelection:NO];
}

I have no idea if this code actually works; I just dashed it off to illustrate the concept.