Uploading the process template only changes the template, not the process you are using - so new projects will have this, but your existing projects need updating.
You need to use the witadmin
command line to update the work item type definition in the project you are working in.
Something like:
witadmin importwitd /f:Task.xml /p:MyProject /collection:http://tfs:8080/DefaultCollection
Will update the WITD in your project.