You can find workflows on myExperiment either by directly searching the myExperiment Web site or by doing a search in the myExperiment Perspective in the Workbench. In either case, you can open the found workflows in the Workbench.
Loading Workflows from the myExperiment Web Site
Say you are currently viewing workflow http://www.myexperiment.org/workflows/1641.html on myExperiment. If you scroll down a bit, you will find a "Run this Workflow in the Taverna Workbench..." section. There, you will a find a URL to the workflow file that you can then use to open the workflow in the Workbench by following the instructions in Loading a workflow from URL.
To open a workflow from a URL in the Workbench:
Loading Workflows from the myExperiment Perspective in the Workbench
Please refer to the section on the myExperiment Perspective on how to search and find workflows on myExperiment from inside the Workbench. A section "Actions on Found Workflows" there explains how to open the found workflows in the Workbench.