newLISP v.10.4.4 on OSX IPv4/6 UTF-8 libffi, execute 'newlisp -h' for more info.
> (import "libc.dylib" "atof" "double" "char*")
atof@952074D8
> (atof "3.141")
3.141
>
Looks like you are using a newLISP version not compiled for extended ffi using libffi. On the Mac use the official version from the binary distribution (currently at 10.4.5), which is compiled for libffi or, when you compile yourself, use a make file with "ffi" in the name like: makefile_darwin_utf8_ffi. Other versions can only use the simple ffi which doesn't handle floats.