one of two things:
either your receiver or motherboard doesnt support 5.1 over optical or your receiver doesnt support the standard (like dts-c, ddl) that your pc can output.
it wouldnt surprise me if your receiver doesnt support 5.1 inputs as this is fairly common (and a big reason i do not suggest htib's) on cheaper home theater in a box (htib) sets though i'd check your manuals for both motherboard and htib to verify.
if your receiver supports it but motherboard does not, a cheap soundcard would work, though sometimes driver updates add the feature (though you say you are already up to date). if your htib does not, then sorry there is no fix for that (other than upgrading the receiver to one which does support it).
in short: first thing to do is read the manuals to check if they actually support 5.1 over optical. the default answer here is "no, they support stereo only" as the only way to get 5.1 is via compressed audio (ddl, dts-c for example) and if both devices do not support this compression you will not have the option.