I'm new to modding and can't seem to locate gameplaydata.package in C:\Program Files\Steam\steamapps\common\The Sims Medieval\Game\Bin. I tried gameplay.package, but that only listed four items when opened in S3PE. Any chance someone could help me out?

You're in the right place, the dll you're looking for is in that package.

Ah, thanks! The guides I've been looking at seem to have skipped the dll step. But since .dlls don't work in S3PE, what am I supposed to do with the dll?

@Dinopuff: If you want to do script modding, you can check out the pure scripting mod tutorial by myself. Be sure to read the requirements, as it assumes that you're using the project template that I've created as well.

I'm looking to create a simple tuning mod for the moment, but I'll definitely keep those guides in mind for when I move onto more ambitious things! c:

Okay, but why were you looking for the gameplay.package if you want to do a simple tuning mod? You won't find any XML/DTTB-resources in there.
All you can do basically, is to inspect the GameplaySystems.dll to figure out which values to modify. If you're not familiar with .NET, that would probably be a bit scary though.

You should be looking in the Delta packages instead.
From the path you supplied yourself, that would be C:\Program Files\Steam\steamapps\common\The Sims Medieval\GameData\Shared\Packages on your computer.

Whoopsy. As I mentioned, I'm a newb! xp

That helps a lot; I think I'm going to be able to do what I'm trying to do now. Thank you so much!

I think I found the XML file I'm looking for, but what program am I supposed to edit it in? It has a bunch of odd characters in Notepad.

EDIT: Tried Notepad++ and I got a lot of "NUL"s.

No problem, I'll help you in any way I can.

EA came up with the brilliant idea to encode most of the XML-resources for TSM into the DTTB-format, and S3PE doesn't understand this format out of the box.
As Peter is not interested in TSM, we had to do something about it ourselves. Fortunately, while very limited, we still have/had some talent working on TSM modding, and velocitygrass came to the rescue back in 2011 with his DTTB-converter and wrapper. And grimreefer24601 have released an updated version of the wrapper.

The wrapper will allow you to inspect the resources within S3PE and the converter will allow you to convert the exported DTTB-resource to XML and back again.

Thanks! That fixed the issue! c:

I think I've just finished my mod! Now to test it. Thank you very much!

Well, that didn't appear to work. I changed the DaysUntilAgeUp value in the bassinet XML to 28, but the testbaby aged up at the normal time.