Carbon absorbs unwanted flavours that are developed during fermentation.
Carbon comes in a couple of formats - loose/granulated or in cartridge blocks. Either are fine to use;
- Loose, granulated carbon will often provided better results, due to generally having more carbon available, more surface area and better control over contact.
- Carbon cartridge blocks are easier to use and less messy - ensure you soak them in water before use.
The choice is yours.
Note: There should be no problem adding the carbon a little late if you've forgotten to add it, provided you will be clearing the wash after fermentation.