Hello Krystyna,
Decauls for the 66 will be very difficult to get, you will have to search far and wide, a service manual for the 66 I don't think exists, I can never remember one being printed but if you e-mail me below I will attach a list of sites.
To put the bobbin case in make sure the take up lever is at the top, move the catch to the right and you should be able to slide the bobbin case in quite easily.
Ken
drivewithken@blueyonder.co.uk, October 2008