ckelley was actually trying to use the Linear Circuit Builder to export a template (class definition in hoc) for a class that, when instantiated, would add to NEURON the equations that describe a circuit. That's different from saving a session file--a session file contains code that recreates one or more GUI objects. When working with NEURON's "builders" (CellBuilder, Linear Circuit builder, Network builder, whatever) it's always a good idea not just to export a template to save a ses file. This will be very useful in the future when one wants to revisit what one previously and try something new.
Anyway, back to this particular error message:
NEURON: no label on cell port
It turns out that, if one uses the Linear Circuit Builder to construct a circuit that involves a model cell, at least one "port" (think "where a wire or electrode is attached") on the model cell must be labeled. Labeling that port makes the membrane potential at that location available to hoc and Python in a special way that plays nicely with LinearMechanism.
When ckelly labeled the port (click on the Label radio button, select Change label, click on the cell icon's "electrode", then enter the name you want to apply to voltage at that point. "Vm" is often a reasonable choice), presto! the next time he tried exporting a template, it worked! And he also saved the Linear Circuit Builder to a ses file, and everybody lived happily ever after.