Ah, that's where it gets a little tricky.  You're not actually specifying a Zenoss-recognized "device"..  instead, you are telling the RenderServer specifically what RRD data file(s) to reference.  Take a look at the "Graph Commands" section of a graph in any performance template.  Ex:

-F
-E
--height=100
--width=500
--vertical-label=percentage
--upper-limit=100
--lower-limit=0
DEF:cpu5min-raw=rrdPath/cpu5min_cpu5min.rrd:ds0:AVERAGE
CDEF:cpu5min=cpu5min-raw
AREA:cpu5min-raw#00cc00ff:cpu5min       
GPRINT:cpu5min-raw:LAST:cur\:%0.2lf%s
GPRINT:cpu5min-raw:AVERAGE:avg\:%0.2lf%s
GPRINT:cpu5min-raw:MAX:max\:%0.2lf%s\j

If you were to use this example (a CPU usage graph), you would need to replace "rrdPath" in the "DEF" line with the full filesystem path to the RRD data in question.  Ex:

DEF:cpu5min-raw=/usr/local/zenoss/perf/Devices/MYROUTER/cpu5min_cpu5min.rrd:ds0:AVERAGE

Aside from knowing the path to the device, you'll also need to know the name of the datapoint you want to graph.  In the above example it's "cpu5min".  Take a look at the $ZENHOME/perf/Devices filesystem tree if you haven't already in order to better understand these references.