There is not yet a built-in task filter to do this, but it is on our list of improvements.

One way to filter by WBS now is the following:

1. Note that this method requires that WBS summaries have task IDs
   • If your schedule was imported from P6, they may not. To create IDs, go to Options>Synchronisation>Primavera P6 and enable Use WBS Code for Summary Tasks before Import or Synchronise From
     
   • To assign IDs in Synchro, select the WBS summaries with missing IDs and select Plan>Reassign IDs
     
     
2. In the task list, right click in the header to Customise Columns
3. Add the WBS Path column to the Selected Columns
   
   
4. In the task list, right click in the WBS Path column header and choose Custom Filter
5. Change the operation to Contains and enter the ID of the WBS summary for which you want to show all child tasks
6. Optionally, Add another row to the filter and choose OR for the Logic to filter by a second WBS path
7. The operation Matches can be used in filters in conjunction with wildcards (* or ? to replace any string or a specific character respectively)