Unless I’m missing something, the only way I can figure out how to multiselect individual controls is to drag a rectangle completely around each control. This works sometimes, but I may not always want ever single control included in the bounding box. Why can’t I do something like “CTRL+click” on various controls? This is useful for me when I need to copy and paste several controls at the same time.
You can select multiple controls by “SHIFT+clicking” each control.
There are other selection options as well, you can find these in the Working with UI Controls documentation here: http://support.balsamiq.com/customer/portal/articles/110078
Hope that solves your problem!