The original bullsquid code referes to 'Mouth' attachment, in this line
GetAttachment( "Mouth", vSpitPos );
in function HandleAnimEvent(...), case BSQUID_AE_SPIT. So it looks up this attachment and builts the vector around it. The model in your link doesn't have attachment named "Mouth"... oh wow, looking at it, it has no attachments at all, well that's the root of the problem.
I guess there's only one way then.
You can decompile the model (using Crowbar), add attachment, and recompile.
Or just grab this.
https://drive.google.com/file/d/0B62rJ8GDQ_WnRXNfVHdrQWwxMUE/view?usp=sharingI've added the attachment and recompiled the model. Use it in conjunction with the one in your link, as it uses that one's animations (it was the easiest way to preserve them, as the decompiler kinda screws them up). So just put both bullsquid.mdl (from gamebanana) and bullsquid_attachment.mdl (from my link) in your mod, it should work.