Hello Leon,
> it does not allow me to change the starting position longitude (as soon
> as I change latitude, the INSERT button does not light anymore and no
> possibility to activate the latutide value edit function)
You are very probably pushing INSERT
twice after inserting Present Position LAT. You should
press it just ONCE (INSERT light will remain ON) and proceed inserting LONG, then press INSERT again (and INSERT light goes OFF).
> the heading is set to a wrong value and it is not possible to change it
Which "heading"?
> the INS selector is not moving, no possibility to set it "on"
Which INS selector? The MSU Mode Selector or the CDU Data Selector?
Please, as you wrote that you've read the supplied documents, try to use the right "INS terminology"

otherwise it's very hard to understand the problem you're experiencing!
Marco