Magento – How to get order Id in class Mage_Adminhtml_Block_Sales_Order_Create_Shipping_Method_Form

First clear the cache.
Then check if the file app/etc/modules/Study_Screen.xml exists. I think it does, but check it to be sure.
I also see there is a space at the beginning of your config.xml: before <?xml version="1.0"?>. remove that (and maybe clear the cache again).

[EDIT]
I've test your code. Here is the problem: in config.xml you have this line

<version>0.1.0<version>

The version tag is not closed. It should be

<version>0.1.0</version>

Tip for the future:
Always develop with error reporting set to E_ALL with display_errors on and mage developer mode set to true. If you do this you will get error messages on problems like this.

$category = Mage::getModel('catalog/category')->load($categoryId);    
$collection = Mage::getModel('catalog/product')->getCollection()
        ->addCategoryFilter($category);
//you can use the next lines to add some minimal information about the products to the collection
//thanks @DavidManners
$collection->addAttributeToSelect(Mage::getSingleton('catalog/config')->getProductAttributes()) //add list attributes
            ->addMinimalPrice() //add prices
            ->addFinalPrice()
            ->addTaxPercents()
//or
//$collection->addAttributeToSelect('*');//if you want all the attributes
Mage::getSingleton('catalog/product_status')->addVisibleFilterToCollection($collection);//only enabled products
Mage::getSingleton('catalog/product_visibility')->addVisibleInCatalogFilterToCollection($collection);//only visible products

Now the problem. Usually the simple products associated to the configurable ones are not associated to a category or they are set not to be visible. If this is not the case for you then the code above should be enough. If they are associated to a category but they are not visible then just comment the last line.
Otherwise you have to loop through the products and when you find a configurable one get it's associated products:

//this is needed if you want to get the out of stock simple products also
Mage::helper('catalog/product')->setSkipSaleableCheck(true);
$allProducts = array();
foreach ($collection as $product){
    $allProducts[] = $product;
    if ($product->getTypeId() == 'configurable'){
        $simpleProducts = $product->getTypeInstance(true)
                ->getUsedProducts(null, $product);
        foreach ($simpleProducts as $simple){
            $allProducts[] = $simple;
        }
    }
}

Now you should have all the products you need in $allProducts array.
This may not be fast due to getting the simple associated products but should get you what you need. I hope it doesn't crash the server.