The block from my custom module isn't showing up on the product page. How can I debug?

It has to be

<codePool>local</codePool>

with a capital p

As for the errors reports, I guess you may need the exception and system logs. Once more, Alan Storm Magento site will come handy Magento Development Environment. Here you will find how to turn ON logging and where to find those log files.

There are other errors that are stored on the var/report/ folder. On my case I have seen only sql updates error to be logged there.

With is there a way to get feedback from the system while I am developing? are you refering to exception handling, logging, etc? Then check this other link: PHP Error Handling and Magento Developer Mode

Magento uses the XML to discover and use different parts of your module.

Sometimes Magento won't say anything if you break the XML, but when you want to use that part of your module, Magento will tell you that it doesn't exist.

One way to avoid XML mistakes (they can be very frustrating to track down sometimes), is to use pre-generated boilerplate XML and class setup files.

SilkSoftware has created a simple "Module Creator" which can save you hours of tedious development.

You can find them here: http://www.silksoftware.com/magento-module-creator/

What they do, is allow you to build the 'skeleton' for your module (without placing any sensitive code in there), they allow you to add blocks, models etc.