pnorm is C code, so the advice in the previous question view source code in R is only peripherally useful (most of it concentrates on functions hidden in namespaces etc.).
Uwe Ligges's article in R news (p. 43) is a good general reference. From that document:
When looking at R source code, sometimes calls to one of the following functions show up: .C(), .Call(), .Fortran(), .External(), or .Internal() and .Primitive(). These functions are calling entry points in compiled code such as shared objects, static libraries or dynamic link libraries. Therefore, it is necessary to look into the sources of the compiled code, if complete understanding of the code is required. ... The first step is to look up the entry point in file ‘$R HOME/src/main/names.c’, if the calling R function is either .Primitive() or .Internal(). This is done in the following example for the code implementing the ‘simple’ R function sum().
(Emphasis added because the precise function you asked about (sum) is covered in Ligges's article.)
Depending on how seriously you want to dig into the code, it may be worth downloading and unpacking the source code as Ligges suggests (for example, then you can use command-line tools such as grep to search through the source code). For more casual inspection, you can view the sources online via the R Subversion server or Winston Chang's github mirror (links here are specifically to src/nmath/pnorm.c). (Guessing the right place to look, src/nmath/pnorm.c, takes some familiarity with the structure of the R source code.)
mean and sum are both implemented in summary.c.