Yes, I think you're right about case-sensitivity being the issue. I'm glad you pointed that out, or it might have taken us a while to guess! The nodes that refer to the library (including Calculate, Calculate List, Make Parametric Grid Mesh, and Make Parametric Points) call it "muParser" when it should be "muparser".
Until we get this fixed, you may be able to work around the problem by renaming the library file:
Right-click on your Vuo Editor.app file and choose Show Package Contents.
Go to Contents > Frameworks > Vuo.framework > Versions > Current > Modules.
Rename libmuparser.a to libmuParser.a.
Go to Finder. (For example, click on your Desktop so that the menu bar says "Finder".)
Hold down the Option key and select Go > Library.
Go to Library > Caches > org.vuo.
Delete the folder whose name corresponds to your current version of Vuo (e.g. 1.2.6).
Restart Vuo Editor. (It will take longer than usual to start since it will need to rebuild the cache you just deleted.)