THREAD CARTRIDGE WONT LOCK IN PLACE

Hi - I'm sorry you have been waiting for an answer to your problem. Can you tell me whether this was first time you used the cartrige, or has it been working up until now? Unfortunately, the cartridges tend to break fairly easily.
To determine whether your cartridge is broken, take out the thread and thread it again. The cartridge won't lock-in unless your presser foot is up and your needle is raised to its hightest position (needle threaders are designed to prevent damage, so they often won't work if the parts of your machine are not positioned properly for threading).  
If the cartidge still won't engage, check the page in your manual that describes the parts that came with your machine - you should see a picture of a spool pin. Although the spool pin is designed to accommodate spools of thread that don't fit in your cartrige, you can use the spool pin all the time. Your manual describes how to install the spool pin.
Fortunately, you can purchase spool pins and replacement cartridges from several online sources, including eBay, and some Babylock Dealers sell them (the Babylock and Brother cartidges are interchangeable). I have the same machine, and I always keep an extra cartridge around. I can thread the extra spool while the other is sewing. When the machine stops for a color change, I just insert the extra cartridge with the next color, then thread up the empty cartridge for the next color. Speeds up your embroidery alot. 
On Google, type in Brother Thread Cassette - you'll get lots of hits, and you should be able to find a cartridge for about $10.00.
If you have and other readers have time to rate my answer (good or bad), your rating helps me determine whether I understand your problem.
If you have more details about your problem or have additional questions, please feel free to comment. I can send you the instructions for threading the cartridge and installing the spool pin if your manual is missing. Just submit a comment - and I'll help anyway I can.
Cheers, Ginny