In order to install fixed version of 2Checkout module, please, follow this steps:
  1. Extract fixed version of 2Checkout module to your stores directory.
  2. Login to your stores admin area and clear all cache (System->Cache Managment)
  3. Logut from admin area and login again
  4. Set your 2Checkout account info on the System->Configuration->Payment methods->2Checkout page. Please, note that secred word should contain only lowercase latin letters and digits.
  5. Setup your 2checkout account accordingly with the screen shots below. Please, dont forget to replace domain name on the screenshots with your one.