1. Categories are also called tags:

First, make sure that your user has the Technical features activated. To do so, go to Setting > Users > Users > Select your user > In the tab Access Rights, check the checkbox "Technical Features". Then refresh OpenERP either by clicking the refresh button of your browser or by clicking on Your company logo in the top left corner.

Second, go to Sales > Configuration > Leads & Opportunities > Sales tags. Here you'll be able to add/delete/modify any tag (aka category). I agree that it is confusing to have 2 names for the same thing (the reason is historical). I will submit a ticket to solve that.

2. OpenERP is unlimited:

There are 2 ways to add fields to a view. An easy way (through the UI) that doesn't provide all the flexibility possible but that could be just what you need for trivial changes (described below), and a complicated way (custom programming in the code of modules) that allows to do whatever you want.

Here is the easy way (which I don't recommend for fields like many2many):
First, you should activate the developer mode. Click here to see how.
Then, select "Manage Views" in the drop-down list. Then, click Edit. The hierarchy of the view shows up. Click on the plus sign where you want to add a field in this view, and follow the wizards.

A few comments:
- You might have to create the related object before creating the m2m field (if it's a new object). You will have to do that in Settings > Technical > Database structure >...
- You should use "Inside" instead of "Before" or "After" for the field position to make sure your view definition survives later updates.

If you are interested to do it in the code instead, the best practice is to take examples on existing m2m fields from other modules.