It looks to me as though the function you are using computes the complementary error function (erfc) for a parameter x, where both x and the returned value for the function are in double precision (equivalent to real*8 as far as FTN95 is concerned).
Neither the complementary error function nor the ordinary error function are standard routines in any version of fortran. Indeed, if you need erf or erfc, you are almost certainly operating in the higher realms of mathematics, and outside the realms of mere mortals.
You have two choices if you want to use these functions: program it yourself (this includes typing in code from a textbook), or buy a library that provides the numerical function and which is compatible with your compiler.
I had a look at the source you gave, and it appears to be a set of files relating to Microsoft PowerStation Fortran v4, which is an obsolete and no longer supported product. There may well be a copy of a numerical library included under license by Microsoft in that system.
I followed the hints in your post, and Visual Numerics (www.vni.com) the firm that produced the library with 'derfc' in it) is still in business.
Eddie