From an invoice, I click Register Payment. However, Payment Method includes only two items: Cash (USD) and Bank (USD). My invoice, journal, and accounts relating to this transaction are all in NGN and that's how I want to record the payment. Where can I add a new Payment Method such as Cash (NGN)?

Payment Methods are Journals of type Cash or type Bank and Checks.

These are managed via the menu sequence:

Accounting --> Configuration --> Journals --> Journals

You can define a currency for each Journal when you set it up, and you need to check the 'Allow multi-currencies' option from the menu sequence:

Settings --> Accounting

EDIT: To address followup question:

To understand OpenERP you need a mixture of 

Experimentation - use the software, try different things

Reading the source code - look at the Python and XML, Activate Developer mode, review patches, use debug mode

Documentation 

Learning from others - use a search engine every time - I do this before I use the forum search

forum posts - including those from the old forums at http://forum.openerp.com/forum/ 

bugs - learn where at https://accounts.openerp.com/forum/Help-1/question/1077

feedback - including http://feedback.openerp.com/forums/77459-general-improvements

Twitter - to understand the conversation others are having hashtag #openerp

videos, blogs, slides

Make sure you know about all of the Documentation sources (and more details on blogs, videos and slides) at https://accounts.openerp.com/forum/Help-1/question/6817

Specifically 

1) The link between a payment method and a journal, it is discussed at https://doc.openerp.com/v7.0/book/3/3_6/invoice_payment.html/

2) That the TYPE of Journal is important, can be found by looking at the XML that defines the dropdown. You can activate developer mode and examine the XML that defines the form view. (The Edit Form View option in the debug dropdown on the Payment screen). You can see the XML definition for payment method is: 

<field name="journal_id" domain="[('type','in',['bank', 'cash'])]" ...