UNIX

    nrnivmodl
    nrngui
Select NEURONMainMenu/Build/singlecompartment

MSWIN

In a DOS box
    mknrndll
    nrngui
Select NEURONMainMenu/Build/singlecompartment

single.hoc

load_file("stdgui.hoc")
create soma
access soma
// area 100 um2 means mA/cm2 identical to nA 
{diam=10  L=10/PI}