First, we recommend you to check if your Mac computer meets the requirements in order to work with the Slingbox PRO HD.
Regarding the PC Desktop application, you need to find the specific location of the IR sensors on your DVR. Once you locate them, you must place the IR Blasters in front of them. This way, you will be able to control the DVR.
The following links will provide you with some steps to apply in order to fix the IR Blasters issue.
Keep in mind that if it is not a supported model, you will need to use an IR Code from a close model number or use a customized code.