Hello Steve,
First you need to create a Performance Counter Metric in the Manage Metrics section to Monitor the Logical Disk Space % Free in which below is a sample screenshot on how to do this.
You can also choose which counter you want to measure, how frequent to poll for this data or which instance you want to monitor.
.
Once you have created the Performance Counter Metric you will now need to create the Rule to evaluate the data that it is collecting. You will do this under Manage Monitor Packs, either create a New Pack and Category or use an existing one. Once you have done this and are on the Rule section where you are adding a Rule you will need to add a Rule Definition using the Performance Counter Metric we just created. A sample of this is shown below which is evaluating the Value being returned and if it is Less Than 20% Free.
Once you have created the Rule then you can add an Action which will allow you to change the Health/Severity status or create a Help desk Incident, Run a Free Disk Script or just simply send an email.
Of course this is a little easier done using version 7.0 but i understand that many users will still need to use version 6 for sometime now. if you need further help you maybe able to request a Pack on the Monitor Community for v6 that may already be done by another customer.
Kind Regards,