Create field default using the ID field in Access 2007

I am setting up an existing Access 2007 database on Sharepoint which assigns
a new ID field. Problem is all our databases have a 6 or 7 digit ID which
match a claim file number. We tried to use the ID field + a 6 or 7 digit
number as a default (= [ID] + 600000) in the claimID field that updates when
a new row is created but Access reports an error "the field [ID] does not
exist etc." I have tried =Nz(DMax("ClaimID","Claimant",""),0)+1 and
=Nz(DMax("ID","Claimant",""),0)+1 as a default. Both result in a error
message that "Nz" is an unknown function. Help!!

Thanks