In order to be able to map a new Rate Plan You need to create the Rate Plan in Hostelworld as Non derived. It will allow to manage everything from Counter
In order to manage the Rate Plan from Counter You will need to be sure it is created in Hostelworld as Non Derived. Once you do that then you will be able to do the mapping from the Channel module in Counter. This will allow you to apply different restrictions per rate plan and everything from Counter.
For Hostelworld you do not need to go to Inbox and can do it directly now from inside Counter :
Channels > Hostelworld> Rates & Availability 
By clicking on NEW rate plan you will be able to add new ones:
Non refundable, Bed And Breakfast...
If you select Percentage then it will be a Derived Rate plan. Counter send the rates to the Standard and HW will make the calculation .
IMPORTANT: if you set as Derived then you wont be able to apply stop sell on it or set MinLos or MaxLos restrictions. 
If you select Amount it means the plan is Non Derived. From now you can put a random rate, no worries because the prices on Counter Rates&Restrictions will update that amount and be the ones online . Then be sure you go to finish the rate plan mapping.
Channel > Hostelworld > Counter Mapping and map each rate plan.
- select the room and active the room (they will pass from grey to green)
- click on rate plan mapping
- and select the rate
If you need any help contact with our support team