The first step is to turn on D2W debug mode in your app Properties file (if it isn't already).
Set:

er.extensions.ERXApplication.developmentMode = true

Walk-through

Advanced D2W Debugging

One of the most common problems to occur in a D2W application is that your rule that you've added just doesn't seem to be processed.
There is a very useful tool to find out what rules are being processed to compute values for D2W keys:

D2W rule tracing.

D2W Rule Tracing

One way to turn on rule tracing is in your app Properties:

log4j.logger.er.directtoweb.rules.ERD2WTraceRuleFiringEnabled = DEBUG

Then say you were trying to debug what rule, or set of rules, were being processed to determing the class string applied to a Diva look component, you set:

log4j.logger.er.directtoweb.rules.class.fire = DEBUG
log4j.logger.er.directtoweb.rules.class.candidates = DEBUG
log4j.logger.er.directtoweb.rules.class.cache = DEBUG

Now when you launch your application and navigate to the page and/or component in question, you will see in the Console output, the D2W rules fired to determine the class.