I think the problem is the way that light bounces off a surface at the same angle it strikes it, like a snooker ball off the cushion. Therefore, if you point it straight up, it comes straight down creating shadows under eyes and chins etc.
With a separate gun, you can obviously modify the angle, fit accessory modifiers etc, and generally get a lot more control. At least that little attachment gives you the option of bouncing which might be handy in a few situations, but it is very limited. The other thing is, and the reason why there are not many of these things about, is that bouncing flash uses a lot of power and the pop-up just hasn't got enough of it to make that a practical proposition.
With that particular modifier, you need a normal height white ceiling (for max bounce light) and to move back in order to create a better angle. The light output will be marginal at best, so raise the ISO and reduce the f/number as low as you can to maximise what you have got (shutter speed makes no difference to the flash exposure).
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