To get debug information from Commerce Server pipelines you should:

Add loggingEnabled=”true” to the pipelines section in web.config

    <pipelines>
       <pipeline name="basket"
                 path="pipelinesbasket.pcf"
                 transacted="false"
                 type="OrderPipeline"
                 loggingEnabled="true" />

Next, set the “_Trace_Discounts” key in the pipeline context dictionary (or PipelineInfo[“_trace_discounts”] = 1;) before executing the basket pipeline. If you are using the StarterSite, you’ll need to edit the code for CommerceComponents.dll.

In ControlLibraryHelpersBasketHelper.cs the RunPipeline() function:

    private PipelineExecutionResult RunPipeline(string pipelineName, OrderPipelineType type)
    {
        // [snip]
        using (PipelineInfo pipeline = new PipelineInfo(pipelineName, type))
        {
            pipeline["_trace_discounts"] = 1;
            // [snip]
            return basket.RunPipeline(pipeline);
         }

The log files are generated in the log subfolder from your pipeline definitions.