Usage of COPY FROM STDIN in postgres
pgScript is a local script extension of pgAdmin, which you most probably do not want here.

pgAdmin is a GUI, not a console application - there is no stdin you could easily use. If you need stdin to stream your content, use psql, which is a console application - with the \copy meta-command of psql.

If you have a file (which you obviously do), just use SQL COPY from pgAdmin:

COPY action_abilitations (id, group_action_id, ...)
FROM 'C:\Users\usernexus\Desktop\database05-12-2012.sql';


The file needs to be readable to the postgres system user.

More info in this closely related request to the pgAdmin support list.