I assume you have already done a proper Forum Search for your model number in case other people have already answered your question ?
If so and that didn't help then check out this thread: https://community.sling.com/thread/3300?tstart=0
Come back if it that doesn't help either.
(NOT a Sling Media representative, just another user)
Can you confirm that your DirecTV receiver isn't in RF mode?
To do this with the original DIRECTV remote, press Menu and navigate to Settings & Help > Remote Control > Remote Settings > IR/RF Setup. Change the setting to IR by following the onscreen instructions from your receiver.
Let me know if this solves the issue.
Product Manager @ Slingbox