I downloaded a vector pack from http://kenney.nl/assets/abstract-platformer but I can't select single objects by Inkscape. Someone had the same problem in this forum, I tried the same steps but it didn't solve my problem. I ungroup the pack but still unselectable. What do I have to do?

After a first "ungrouping" you need to go to "Edit->Clone->Clone linking->Disconnect (alt+shift+D) - not sure about the English terms in this case.
New groups appear- You can ungroup but will notice objects are combined by same color for instance. You can get smaller chunks by going path->disassemble (shift+cmd+K)

Cheers
P.

Thank you Polygon! It's still difficult to get a single object but at least they are ungroupped and selectable.