So if I have Aard skill for example do I have to add each tier to a skill slot?
I added just aard intensity (tier 3 1st point) to a slot but then did not get aard sweep?

Where is this explained?

yes in order to use a skill it has to be in a skill slot, this is explained to you the 1st time you pick a skill & you can also find the explanation by opening the Glossary & click Tutorial, then click Character & then read Character Abilities.

thanks