If your in the USA, I dont think the Slingbox Pro has an ATSC tuner. Its Analog only so there are no longer any signals for it to find. The USA has been 100% Digital for a few years now,
The antenna (or aerial if you prefer) you are trying to use works with the diguital TV because the digital tv has a built in ATSC tuner.
Their are many digital set top boxes available that will receive the digital ATSC signal and outut to the Sling. Just treat the Slingbox as an older analog only TV.