How to create monsters
 

I've been trying to create the Fiendish Mustard Jelly from Rise of the Runelords and I just can't seem to do it with Hero Labs? Is it possible, or is this a house special???

No need to reinvent/recreate the wheel, the Rise of the Runelords stuff can be found in the Community Bestiary files as discussed in this thread and found here.

Is there a specific file that enables the others? DL'ed the Runelords files but when I tried to import one it was a mess.

Can you be more specific on what you mean by "was a mess"? Did you receive an error? (You can right click and copy the error message to paste it here, and that will go a LONG ways in letting people help you.)

As was linked above there is two steps you have to do.

1) is to download and install the Community Bestiary package by following the setup instructions.

2) Once downloaded and installed you can then download any of the .zip files, under the "Community Bestiary NPCs", from d20pfsrd.

Without doing both steps you will have issues.

If you did both steps and still have issues then please do as Umarian says and we can try and help. :)

The fiendish mustard jelly is in the por file 5 - Sins of the Saviors/Part 3 - Runefore and the Abjurant Halls/E4 - Befouled Chamber.

It uses the mustard jelly from the community Tome of Horrors Complete, not the one from Frog God Games. That might be were the issue is.

Thanks for the help. I used HL in my campaign for the first time Sunday and it speeded up things tremendously. Wish I had had it sooner.