That is on page 51 under Skills:
"The effects of the following Skills and/or bonuses are immediate
and may be used the Turn in which they are acquired.
This means that if one Action causes a Survivor to level up
and gain a Skill, that Skill may be used immediately if the
Survivor has any Actions remaining (or the Survivor may use
any extra Actions the Skill grants)."
I knew I had read it somewhere...just had to find it.
This should have been put in the first paragraph that talks about Experience, I've had hard time finding this too