From 4.6c onwards, Field exits will no more be supported by SAP.  They removed the function of field exit but they had given lot of flexibility through userexit.

However, if you still required it, here is how to activate it :-

First called up transaction  CMOD.
Then called up transaction PRFB.

or

Activation of the field exits and assignment of the dynpros can also be carried out using program RSMODPRF. For this purpose, the program must be started without parameters (input fields remain blank).  If required, new field exits can be created using program RSMODPRF (see the program documentation).

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SAP ABAP/4 Programming, Basis Administration, Configuration Hints and Tips