I tried the website you must have been trying ; https://highway-buses.jp/ticket/matsumoto-takayama.php
and found out that it's not working properly.
I made a phone call to the bus company but they don't know what's going on, so they will check the company which operates that website.
BTW, your ticket is available without internet.
But there're a few steps you have to be done.
Let me tell you one by one.
1. Book your round trip bus ticket first.
* DO NOT finish your payment. Just book the seat.
2. Bring that information to "Central Honshu Information Plaza in Keio" by the day you will aboard.
*Your ticket details are from here, please check here out and make sure the details
3. Show your round trip bus ticket information and ask " Three-Star Alps Unlimited Ride Shinjuku Pass Type-S2" to purchase.
They will change your ticket to Three-Star Alps Unlimited Ride Shinjuku Pass Type-S2 from round trip bus ticket.
Please finish your payment at here.
Or, this is a suggestion from the office, you can make a phone call to +81-5990-5115 to finish all of your booking including Three-Star Alps Unlimited Ride Shinjuku Pass Type-S2.
They have some English operator during the office hour.
You only have to tell them that you want that pass, and also the buses you want to take.
If you travel during the holiday season in Japan ( approx 1/1~8/1) it will be always commuted and the bus tickets would be sold out very soon.
So either way, you should better and I strongly recommend you to book your bus ticket as soon as possible.
Hope this can be your help, and enjoy your trip!