The tuner on the US version of the Sling Classic is NTSC-only as far as I know. That's likely your problem. Here's a clip from the user guide:
(Click to Enlarge)
If you have something that has a PAL tuner and an A/V output (composite or S-Video), you might try that with one of the other inputs on the Slingbox. A VCR perhaps ?
Hope this helps,