I mixed my 10g priming sugar with a little boiling water to create a sugary solution, dumped that through a funnel into the minikeg, then syphoned my 5l of ale in on top.
I wonder if you had a good seal with the bung - I applied a little vaseline to the ring of the bung which connects to the keg and also a little vaseline around the punch plug in the middle of the bung - just to try and avoid getting any leaks while it carbs up.
I have read that using regular (bottle) levels of priming sugar can cause the keg to deform due to too much pressure while it carbs up.
Just my first mini-keg so early days but based on this one 10-12g of priming sugar (approx) seems about right if you get a good seal during carbing.
Hope this helps.