Since you have responded to
the thread discussing iPad cover support, you know that there needs to be a special <meta> tag in the metadata section of your OPF file.
The currently available version of Sigil doesn't not have this functionality since the iPad was just recently released. The next version of Sigil should have this.
For the time being, just create your epub file as normal, and then extract it with a zip tool, add the required <meta> element by hand and zip it back up (do note that the mimetype file has to be the first file in the final archive and also has to be uncompressed).