To add a new payment method you should add a new journal (accounting > configuration > journals). Use as an example the journal 'bank' - or just duplicate and change it to avoid mistakes.

To use an existing payment acquirer for manual payments registering, go to the same menu journals, find a required on, chck the box 'For Incoming payments' > manual (the tab Advanced settings > Payment method types on a journal form).

UPDATE FOR ODOO 8

In Odoo 8 you need to create a new journal for payment acquirers. As a type you should select 'Banks or Checks', and link this journal to proper default debit and credit accounts. You may use the same as in the "Bank journal" if received money is put there, or create new accounts. As an example use the 'bank journal' (you might just duplicate it, rename, and optionally change accounts).