Modern Symbolic debugger for R programmes?

Worik R <worikr at gmail.com>

WR> I cannot make setBreakpoint or findLineNum  work.  I get "No source
  WR> refs found.".  I am starting to loose my mind!

Indeed, as Rainer suggested, check out ESS. The ess-tracebug (part of
ESS) provides, among other things, source level debugging, breakpoints
and versatile watch window. A bit old but still relevant documentation
and screen-shots can be found here
http://code.google.com/p/ess-tracebug/

    Vitalie