That wasn't very clear either, but I don't blame ya, because there isn't a straightforward answer to the question.
If you open a MLD file in a Hex editor, you will see reoccurring patterns of data. If you know how to interpret these bytes, you
will get the models out.
A very simple example: the level data of Pocket Bomberman. One byte = one block. If you read 256 bytes from the ROM address 0x2403C, it is very obvious for anyone who has played the game that this is the level data of world 1 level 1.
Because the game is so simple and the data is only one-dimensional, you can immediately see that those seemingly random bytes are, in fact, the level data. You don't even need to write an interpreter because it's so obvious. There isn't a documented "format" for the levels, nor a "reader" or an "editor" but you can still see and edit it all in a hex editor.
Even if you're in doubt, you could always edit a few bytes and play the game and visually SEE your changes.
Thus, once you've figured out exactly how the level format works, you pretty much know how every other level works too.
When it comes to 3D-games, though, "seeing" the data isn't as easy, but a few things still hold true, was the game for the Game Boy or the PSX:
-reoccurring patterns hint at a relatively simple structure
-editing the files will result in changes in the game
-once you've figured out how one thing works, it is very likely that all the others follow the same scheme
Some people might be so good at identifying data that they could immediately recognise a 3DS or a DAE file just by looking at its bytes. I'm not, so I can't really help you. But if you get what I'm after, you could map out the model format with enough determination and time.
Try editing a few bytes, see how that affects the models,
always take notes and eventually you might publish a document titled "Tiny Toons Adventures MLD model file format specification"