I knew this would be challenging, but the worst part of getting a 32-bit ISAPI DLL running in the 64-bit version of IIS 7 is the really crummy UI.  Here are the steps:

Allow the ISAPI DLL

1. With the root node selected (the computer name), double-click Isapi & CGI Restrictions. 
2. Click Add... on the sidebar
3. Type/browse for the path to your ISAPI DLL.
4. Check Allow extension path to execute.
   
   

Create a 32-bit Application Pool

1. With the Application Pools node selected, click Add Application Pool... on the sidebar.
2. Type in a name and choose the Classic pipeline. 
3. Opt not to start the pool immediately.
4. Click OK.
5. Select the new pool.
6. Click Advanced Settings... on the sidebar.
7. Change Managed Pipeline Mode (General category) to True.
8. Start the pool.

Create the Application

1. Right click on the Web Sites > Default Web Site node and click Add Application...
2. Choose the alias, application pool, and physical path.  The application pool you choose should be the 32-bit one you just created.
   
   

Allow ISAPI DLLs to Execute within the Application

1. Select the application node in the tree view and double click Hander Mappings.
2. Click Add Script Map...
3. Type a request path of *.dll, select the IsapiModule module, and give it a name.
4. Click OK.
   
   

Note:  You can also enable the existing ISAPI entry at the Default Web Site level, or enable feature delegate for ISAPI Filters at the root node (computer) level and then edit it at the application level.