set the "Model path/name.mdl (mdl)" parameter of the info_tfgoal to: "models/backpack.mdl".
you need to tell the info_tfgoal what model to display or it will be invisible as you found out. heres a tut that shows all the settings to use:
http://tf.valve-erc.com/kbase.php?id=36if you plan on making a tfc map you should definately join the these forums:
http://tf.valve-erc.com/ and use tfmapped to find all your tfc answers. very few people here map for tfc, but everyone there does