The MultipleHash.snk file is the Strong Named Key file for the project. As the author I have chosen not to release this file into the public domain, which is a common practice (see all the Microsoft SSIS code releases).
To create a new SNK file...
Open the project in VS 2008
Select Project Properties (for the 2005 or 2008 project)
Select the Signing Tab
Generate a new key.
Once this is done, you will need to compile, and then get the Token (use the command sn.exe -T MultipleHash2008.dll) or search on Google for "get publickeytoken from dll"
Then update the line
UITypeName = "Martin.SQLServer.Dts.MultipleHashUI, MultipleHash2008, Version=22.214.171.124, Culture=neutral, PublicKeyToken=51c551904274ab44",
such that the PublicKeyToken matches that of the new DLL, with the new SNK.
Recompile, and reimport into the toolbox in Visual Studio, and you can use your modified version.
If you are finding bugs or issues, please let me know so I can apply the changes...