You can refer to the network that you mentioned, also in the toolkits menu (top of network editor) Fwd/Inv toolkit, potential_based_bem, torso_tank_bem.srn.  If you get the SCIRunData path set properly, this network will run and help you.  Download the SCIRunData directory (same location as SCIRun), and find the configure menu button on the bottom of the editor.  choose the options tab and type (or copy and paste it) the path to the SCIRunData directory here (include SCIRunData with a / at the end).  make sure that you press set and (re)load the network.  Once you have that working, you should be able to see how the network is supposed to work.   You will be able to run any of the example networks too.

As a quick run through, the BEM forward problem is based on BuildBEMatrix.  It takes surface meshes and compute the BEMatrix.  Put the heart surface inthe first port and the torso surface in the second.  Set the properties in the ui, the heart as the source and torso as the recording surface.  make sure the conductivity is set properly.  Multiply the output of the BEM matrix with the matrix of source potentials using evaluateLinearalgerbraBinary, then you can use getroworcolumnfrommatrix to extract a column and visualize it.

To map the torso surface potentials to a subset of points, you a can use mapfielddatafromsourcetodestination, or mapfielddataontonode/element.  the destination (subset) is the second input and the potential field is the first.  you can also use BuildmappingMatrix (similar to mapfielddatafromsourcetodestination) to make it faster for multiple time steps.  Another way is to just take the columns for the subset out of the BE matrix, as long as they are a true subset.  

cheers,

Jess