The button to open/close the flags widget is on the bottom right corner, with an icon of a flag. Clicking this button will cause a small window to appear, showing the flags which are relevant to the current process.
Example: if valkyrie --view-log=log-file was given on the command line, then this window would show:
--view-log=<log-to-view
Example: if valkyrie /bin/ls -lF was given on the command line, then this window would show:
/path/to/valgrind --tool=memcheck --xml=yes /bin/ls -lF
Note: Valkyrie only sends flags to Valgrind which are not the defaults.