as a rule of thumb - a mess on the underside is a top tension issue. Put on two different colored threads and you will know which one is the problem quickly. And if you don't have it threaded quite right -that is the normal result, a rat's nest on the underside.
They are very fun machines, I'm looking to get one just because I love all the attachments you can get for them. Passed up two that DH said were not nice enough for the price.