STEP 1
Go to transaction code KKPHIE and enter the cost object (usually the plant code)
STEP 2
Double click on the cost object group in which you want to assign the material,
Then click on 
STEP 3
Click on 
and then on 
to be able to add new materials.
and then on to be able to add new materials.Add the respective material and then go back 
and click save 
.
and click save .