Here is the state of iPad/Numbers/Open Office/Excel Compatibility.
HeroForge is an Excel 2003/2007 file be default.
It works fairly well in Excel 2000, but your mileage may vary.
It should work pretty well in Excel 2002 (I think 2002 and 2003 are the same, but I am not 100% sure about this)
It may or may not work with Excel:mac 2004, but it is not officially supported. Your mileage may vary.
It is not compatible with Excel:mac 2008 at all.
It is not compatible with any other spreadsheet program out there. Not a single one of them. (This means it will not work on your iPhone, iPod Touch or iPad)
The reason behind this is quite simple... VBA.
HF in its current state uses VBA, and not just any old VBA, but the VBA version in Excel 2003/2007. If the sheet calls function that aren't in the VBA Engine that your Excel version uses it will enter Debug Mode, instead of working.
Non Excel spreadsheets don't support VBA. (Open Office does, technically, but the implementation is horrid such that I have never seen it actually work.)
Excel:mac 2004 uses the Excel 97 VBA engine.
Excel:mac 2008 has no VBA engine.