All-caps is tiresome to read, consider going to lower case default.
You forgot to insert rows into the movieactor table.
A query first collects its data, then assembles the requested result. Build your query similarly, first the From clause ...
from moviaactor ma
join movie m on m.id=ma.movieid and m.name="Big"
join actor a on a.id=ma.actorid
where m.title="Big"
... then prefix the desired Select expressions ...
select
group_concat( concat(a.firstname,' ',a.lastname) )
as "Actors in 'Big'"
from movieactor ma
join movie m on m.id=ma.movieid
join actor a on a.id=ma.actorid
where m.title="Big";
(I've no idea who's in what pop movie, I just added arbitrary ids to get it working.)