To add Adjustment Reason Codes go to Advanced Setup-> Claim Lookups.
On the Search Claim Lookups screen, click on the 
button to add a new Claim Lookups.
A new claim lookup window opens up for configuring adjustment reason codes.
Enter the Type by selecting 
and choosing ‘Adjustment_Reason_Code’ from the list.
Enter the new reason into the fields named – Code, Code Description, Value, and Value Description.
Keep the User Editable field checked, if the value needs to be edited later.
Hit the 
button to add the lookup value.