This little How-To will walk you through how to go about threading the mast of the Brother Convertible. This is also just about the same for the LK150 and several other plastic bed hobby machines.
Place yarn on floor or on the table at the base of the mast. Put yarn end UP through eye of triangular guide.
Pull the metal disk outward and you will see the metal shaft of the tension unit and you will also see a little white knob behind it. The yarn needs to go BETWEEN those two. It's very easy to slip it in there.
The arrows show the sequence of the thread up through the mast tension dial unit, out toward the front and through the little metal loop then it will go up to the loop at the end of the wire. See the next picture.
Here's the end result of the mast threading.

After the yarn is through the metal loop at the end of the wire, bring it down and thread through the machine.

Be sure the yarn is securely behind that little white > part in the carriage.