A little too late, but I found the answer to your question. The offer of the free Plus was offered up to and including the 21st of July, so it may have been still visible for you due to caching. Apologies for the confusion. Enjoy planning your trip!  

Thanks to all for the help! I changed my mind to a global pass. The Option was there yes, but not for free… I can’t understand why its costs 23 Euro, and its advertised as special free offer. 
I have to say it makes the process a lot more complicated and it is not very intuitive. But the pass is bought now, i skiped it. 

Hi @Quen Tin, you can't find the option, as '’Plus'’ is only available for promotional Passes. You have selected the '’standard price option'’ for the One Country Pass Germany, so it won't give you the option to choose '’Plus'’.

With promotional Passes the '’plus'’ option is shown on the '’order details'’ page.
 

1. Order basket → click on '’checkout'’ button
2. Review my card page → click on '’proceed to checkout'’ button
3. Order details page → add '’Plus'’ to order 

PLUS is even possible for Standard passes Upgrade from 85% Refund to 100% Refund :) 

@Nanja maybe can help where to select the Plus Upgrade :)

No. For normal passes, you can get a refund (before the first day of validity) anyway.

Ok. I red about the Plus on the InterRail Website. Its says that at the moment, its possible to upgrade to PLUS for free (at checkout). 

My pass ist not a discountet pass, its a one-country-pass for Germany. Normal price. 

Is PLUS available for this pass?

thanks for the anwser! 

“Plus is only available during promotions.”

https://www.interrail.eu/en/interrail-passes/ordering-info/plus

Are you trying to buy a discounted pass? If so, which one?

OK thanks. But i still cant find the option….

The Plus upgrade is before the payment process :)