1. Open your OF (PES2010EDIT.bin) with PES 2010 Editor and double-click the player you want to add a face for. As you can see on the image, the face 'mode' is set to build (can be 'default', too); this means you have to select a new face slot for the player. Click on 'relink' to do that. If the face mode of the player is already set to special, go to step 3.
2. You will see a list like this. Click on 'Free Faces' so the editor shows only unused faceslots. You can select any faceslot (apart from 258, which will make the game crash) by double-clicking on it.
3. Face mode is set to 'Special' now and you can see 2 numbers, for the face and the hair files
4. Download the face you want to insert and extract the files. Normally the files are named 'Merida_face.bin' or 'Merida_335.bin' or something like that; The lower number is the face file in that case. Rename the face and hair files according to the 2 numbers you can see in PES 2010 Editor and copy them to 'PES 2010\kitserver\pesedit\img\dt0c.img' *If you are not sure which of the files is for face and which for hair, you can open the bin files with GGS 7.4, you will see face or hair textures then.