Can't add items to vanilla creative tabs =(

[reallydrained]

I have tried adding my 2 items to the vanilla Materials tab in creative in almost every way imaginable but it won't work. I tried:
"creativeTab": "itemGroup.materials"
"creativeTab": "Materials"
"creativeTab": "itemGroup.Materials"
"creativeTab": "materials"
"creativeTab": "MATERIALS"
"creativeTab": "itemGroup.materials=Materials"
"creativeTab": "CreativeTabs.tabMaterials"
"creativeTab": "tabMaterials"

The correct lang term is itemGroup.materials, it shows up as itemGroup.materials=Materials in en_US.lang file. When I tried making a tab like this:
{
"creativeTabs": [
{
"tabLabel": "itemGroup.materials"
}
]
}
it made a new tab called Materials:

I am just confused here, is there no way to add an item to a vanilla cretive tab? My only other alternative is just having them only accessible via NEI which would be okay I guess but I'd realllly like to add them to vanilla tab not a new one or hidden completely. I am not adding a ton of items so it makes no sense for me to add them to a new tab but I'd also like them to be accessible. I have looked through all of the documentation and it doesn't seem to adress this. If there's any help you could provide I'd be very greatful! Also, I tried the 1.0.10 alpha version and it didn't seem as stable, I think I had some issue so I am using 1.0.9. Both versions had this issue though.

MC version: 1.7.10
Forge: 10.13.4.1614 (latest)
CustomItems version: 1.0.9 beta