I have the following fastload script
Have you tried DEFAULT instead of :DEFAULT?
Also, since you have defaults assigned on the CREATE TABLE, if you leave out the column references from the INSERT and the VALUES clauses, it should work, as in:
INSERT INTO "TABLE" (FIELD1, FIELD2, FIELD3, FIELD4) VALUES(:MITS_CLAIM_ID, :PLAN_CODE, :MBR_MEDICAID_NBR,:DTE_FIRST_SVC_HDR);
That should enable defaults to be applied to the LOAD_USER and LOAD_DATE columns.