What's your system menu version?
older menu used older IOS slots.
60 or 70.
some softmods guides are even patching IOS60 and installing the result in all three slots 60/70/80.
I never patched it myself (I'm still on 3.2E, I don't have the SD menu), so I'm not expert in this kind of hack.
Maybe someone else could confirm what patch you need to do, and which IOS you need to use.
Or just update your softmod with modmii, it will install all required IOS.