After installing the PDMWorks Client you should specify a common work directory. This is the directory where PDMWorks will copy all of your files you open from the Vault. Most common are (C:\Working or C:\WPDM Working).
You will need to go into your Client options by right-clicking on the Vault Pane and picking PDMWorks Workgroup Options. On the "Folders" page there is an area to specify the location for My PDMWorks Workgroup Documents. Select "Use Folder" and browse for a location to store these files in.