In order to examine a program's internal state, program execution must be paused. If your program is currently running and you wish to manually pause the debugging session, select Debug | Pause. All active threads will be suspended until execution is resumed manually.
When paused manually, BugSeeker arbitrary selects a thread to be the current thread. It is the current thread whose stack frames you will be seeing. To make a different thread the current one, ensure that the Threads debug tab is visible (View | Threads) and double-click on the thread that you wish to make the current one.