How the field 3600 (Retained earning) is entered/calculated?
Field 3600 (retained earning) on balance sheet GIFI sch100 (or sch101) is a calculated amount. 
Go to page 6 of sch100 (or sch101), first you will need to enter an initial amount of retained earning start for the tax year (field 3660); then go through the rest fields on that page, it will come to retained earing end for the tax year (field 3849). Finally field 3849 will go to field 3600.
In short, Field 3600 of sch100/101 is calculated from page 6 of that form, please follow page 6 calculation; finally the amount in field 3849 will go to field 3600.