Cant slide the needle plate setting knob over for rolled hem

If you dont have a manual then this page will allow to to download it for free. Pretty straightforward and easy to read
http://janome.com/siteassets/support/manuals/sergers/inst-book-634d.pdf

http://content.janome.com/userfiles/file/inst%20book%20634D.pdf

Change the needle plate setting knob to rolled hem, starting on p. 28

here is a link for parts for the machine and also a manual for the
machine

http://www.sewingpartsonline.com/Janome-644D-Sewing-Machine-Parts.aspx

Push blade from right to disengage...slide roll hem ****** forward...reverse operation to return to ordinary sewing.
It is covered in the manual, do you need one ??

Have you tried youtube for 'how to' videos? I just found this one that might help. Good luck. http://www.youtube.com/watch?v=JyrAhSQAEeY

You need to fit a rolled hem plate

try you-tube to video a video

go to this link and i think your manual is the first one on the page.
Janome Mylock 234D overlocker Google Search

You need to disengage the cutting blade by openingfirstly opening the front and left hand side covers. pull on the larger of the two silver knobs and slide the red lever towards you. I'm not sure what the tension settings are, but I'm certain you need to reduce them a fair bit to aid in the rolling effect. I only use 3 threads when doing a rolled hem.
My description isn't too detailed but I hope it helps!

This may be too small to read, if so, email via
www.bargainbox.com.au and I shall send it on........remember to only ever use the R needle, so you only have 3 threads.

To move the Rolled hem arm, you will need to release the upper knife (knob on left inside cover....push to the right and blade moves approx 6mm) so you can slide the small metal tongue to correct position for:

N = Normal
R = Rolled