Right-click on a block and click on Explorer. On the page that comes up, in the Model Hierarchy, there will be a node for "Code for ..." (e.g., "Code for vipmosaicking"); click on that. That should allow you to see any associated standalone code.
If, though, the block is a MATLAB Function, then there will be a node in the hierarchy entitled "MATLAB Function": right-click on that and select Open. Alternately, if you know that a block is a MATLAB Function, then right in the graphical model view you can right-click it and select Open to be taken directly to the code.