RE: How do I configure GP 2013 Web Client to load modified forms dll
There is a word document in the SDK installation folder that gives you a hint to the dynamics.exe.config. Took me a while to notice it even though it is called "Readme".
Basically I created my assembly with the name of <mycompany>.<myteam>.<formtomodify> so that would come out like CompanyA.TeamA.VendorMaintenance.dll when compiled. The class that is generated is the GPAddIn default.
So you need to add this dll to the main GP folder and not the addin folder.
I took a copy of the dynamics.exe.config file to a separate folder so I could keep copies for version control. Also it doesn't like you editing it in place. Once there open it with notepad or notepad++. If you want the addin to work in both full and web client you need to add the <addin /> tag in both the <Dynamics> tag at the bottom of this file and the <DynamicsGPWeb> tag. Note that the end tags for these Dynamics ones are way at the end of the block of text for the Dynamics main files. I just moved the end tag to its own line and added mine above it.
So for my above example the tag to add in BOTH places is:
<addin name="CompanyA.TeamA.VendorMaintenance" type=''CompanyA.TeamA.VendorMaintenance.GPAddIn,CompanyA.TeamA.VendorMaintenance"/>
The name bit at the front is whatever you want it to be. I used the assembly name for my just to keep it unique. The first bit of the type is the fully qualified name including class. The second bit after the comma is the assembly name without the dll file extension.
Hope that helps. Sorry I took so long to reply.