That should be all you need be doing in the Designer. Create a function model, and create function definition and use it in the other view transformations. The only thing specific you need to do is when you are building the VDB, you need to explicitly add the function model to the VDB, it is not automatically added. Also deploy the jar to the Teiid/Jboss AS server.
If you want you can attach you VDB, we can take look to see what is wrong. Also what version of the Designer you are using?
I am using Teiid 7.1.0. I had not added the model to the vdb, since I just wanted the query to be properly validated in the transformation editor even before adding it to the vdb. Also do we have to change the way we are calling the function in the query? Previosuly in MMX we used to just write the function name in the query. But now since we have a model (.xmi) for the function do we have to access it by giving a fully qualified name?
In Teiid Designer 7.1, your custom functions should be available in the Transformation Editor's Expression Builder dialog. If not, you can try restarting Teiid Designer which should load all existing custom functions into Teiid Designer's function library.
In the upcoming 7.4 release, we've fixed this issue and improved the user experience for Function models including adding them as dependencies to View models AND to automatically add them to VDB's, similar to other dependent models. See: http://issues.jboss.org/browse/TEIIDDES-941
We'll have another milestone available next week for 7.4