gardenline strimmer line?

Hello,
I've got a new gardenline strimmer from Aldi & after just 2 uses the line has run out. Can anyone please tell me how to re-wind replacement line onto the spool, as it has an outer casing of white plastic so you can't actually see the part where the line is supposed to wrap around.I should have said that I got the spool off the machine OK but the line spool itself is cased in white plastic.
Thank You;
Derek