All aeronautical frequencies are in the database here, and the museum uses the WPAFB trunked system which is also listed.
Plug them all in and you should be good to go.
I just updated the freqs a few weeks ago, so they should all be accurate. Here's a little more detail:
The B-25s will arrive at Grimes Field (I74) in Urbana on April 15th. They will practice flying on April 15th-16th out of I74. You should be able to go there and watch (be sure to stop and see the B-17 restoration project!).
They will be using an interplane frequency to communicate within the flight; probably somewhere between 122.750-122.775 or 123.125-123.575. They will also use 122.700 (I74 UNICOM) and 118.425 (Dayton Approach). In the chance that they switch to Patterson's airspace, they will be on 126.900 (KFFO Tower).
On April 17th, the aircraft will land at the museum field, which is controlled by a special tower frequency, 123.225; they may use 126.900 as well.
On April 18th, they will take off from the museum field and fly overhead in formation; this will probably be on 123.225 or 126.900, as well as whatever interplane frequency they have.
I'll try and remember to post what I hear if I can pick it up.