Serial number located

take to a local repair shop

make sure that the bobbin is place in the holder so that the thread comes out in the right direction

You may get help for that on the Janome web site, the bigger manufacturers often have a support page or download instructions

Here is one set of Reviews.

Janome Embroidery And Sewing Machine Reviews

try this link
Janome Accessory Tutorial Videos

Yes up to 4mm wide twin can be used

have you tried changing the needle?
have you tried adjusting the top thread tension?
it might be the needle and the thread are not compatable,, the needle may be to light for the thread.
if the suggestions don't help, sew on the sewing machine till it makes the loop and then stop the machine and take it to the repair man and let him see the loop and then ask him why it's making the loop.

Look on the back side for a slide type switch.

On any movin g poart that you can see or get to.