Make sure that your custom module is in the addons folder if it isn't already.  Then make sure you have the __init__.py, __openerp__.py, and any other python or xml view files in your module folder.  Ex:

product_custom_module_folder:

    > __init__.py

    > __openerp__.py

    > product_custom.py

    > product_custom_view.xml

In the __init__.py file write the name of all the python files in your module without the .py file extenstion like this:

import product_custom

In the product_custom.py file you can include your custom field by inheriting from the products model like this:

class product_product(orm.Model):

    _inherit = 'product.product'

    _columns = { 

            'custom_field_1': fields.integer( 'Name of custom field'),

            'custom_field_2': fields.char('Name of other custom field'),

    }

In the __openerp__.py file make sure to include the following:

{
    'name': 'Custom Product Module',
    'version': '1.0',
    'summary': 'Custom Product Module. . . ',
    'description':'''
        Custom Sale module to add additional fields without modifying the original Sale module
    ''',
    'category': 'Custom Products',
    'author': 'Your Name',
    'website': 'http://www.yourwebsite.com',
    'depends': ['product'],
    'data': [
      'product_custom_view.xml',
    ],
    'installable': True,
    'auto_install': False,
}

Note:  You will only need to add the xml file if you plan on creating new or modifying other views

Once you have all of those files in place, then go to Settings > Update Modules List and click the Update button

Then click on Installed Modules, and clear the "Installed" search filter in the upper right corner and type in the name of the new module.  When you see it, you can click Install and then your custom field will be added to the products table.  Hope this helps