I didn't read all of the comments, so forgive me if I am repeating what others have said...
IF there is a sewing needle in the machine already, you can feel where the slit is in the sewing machine needle. Just use your finger to feel. If you are not sure what I am talking about, take a sewing machine needle from your pack of needles and look closely at it. You will see a groove that goes down the length of the needle: this is the side that you will thread your needle from.
Another way is to take a mirror and look up where the needle is inserted. This will show you which side the flat part of the needle needs to be inserted. Since the flat side of the needle is the opposite side of where you put your thread in (the side with the slit), you will know which side you thread the needle from.
Hope this makes sense.