Full debugger support in BlackPearl for setting breakpoints and such is on its way.  In the meatime, save yourself some manual debugging hassle by emitting messages to the BP Console with this code:

string loggedInfoSource = string.Format("{0}/{1}", K2.ProcessInstance.Process.Name, K2.Event.Name);
K2.ProcessInstance.Logger.LogInfoMessage(loggedInfoSource , "Your helpful status message");

Note: You must run BPService in console mode to see these messages.  Be sure to run the BPService executable under priviledged credentials with "Run As ..."

Hate having to crack open the InfoPath file just to get the appropriate namespaces for your XPath queries?

System.Xml.XmlNamespaceManager nsm = ...; // ...
nsm.AddNamespace("my", "http://schemas.microsoft.com/office/infopath/2003/myXSD/2006-04-20T16:26:21");

Use this little gem instead of the second line above:

nsm.AddNamespace("my", xd.DocumentElement.GetNamespaceOfPrefix("my"));

This line lets you skip that annoying manual step and get straight to the querying!