Hi
You need to open and activate the hierarchy for both thesauri.
Go to Thesaurus->Hierarchy
Find if you have the records for the chronological thesaurus, searching the TLD = dc
( in version 5 the component is TLD(Alpha2) )

If you don't have it, create new record and fill as:

Follow the screen instructions, for v5 will show a «ok» or «is ready», in v6 you will see a confirmation window with all the information:

The thesaurus will be ready to be use in the thesaurus tree.

Done!
For onomastic is the same but the TLD = on
You can see al thesaurus TLD's here
I hope is clear.
Best