A proven method for learning any new machine - sewing, embroidery or serger, is to not read the manual first. Unless you have insomnia!
Instead, open the manual and lay it right next to the machine and then read and apply each step of each feature from bobbin winding to maintenance (except for the serger, which does not have a bobbin).
The pictures in the manuals are mostly for reference and should not be the sole method of learning as important steps can be missed.
For the bobbin issue - the bobbin thread is suppose to pull the top thread to the back of the item being embroidered. Be sure to use the correct weight of bobbin thread for machine embroidery.
Most bobbin thread is 60 weight. Seventy would also be okay, but 90 wt (if not specified for this model) would be way too thin,
Thread weight and needle size can get confusing:
Thread - the higher the weight number the thinner the thread. The weight along with thread type (poly, rayon, cotton, silk) and yardage will be marked on the spool end.
Needles - the higher the size number the larger the needle. First number is European and second is US. The numbers are sometimes interchanged. When a size 90 needle is recommended, it is the same as a size 14. The needles and packages are marked with both numbers as well as what type (embroidery, ballpoint, jersey, denim, sharp, quilting, top-stitching, microtex and more).
For machine embroidery, you want sharp for most items and ballpoint when embroidering knits.
I prefer titanium coated as they can be used longer and are non-stick.
http://allstitchembroiderysupplies.com/department/15x1-pd-flat-shank-regular-eye-titanium-10141.cfm Be sure to purchase needles for the home embroidery machine and not a commercial machine.
Best wishes and have fun embroidering everthing you can get your hands on (start with old garments for practice).