With the where clause, getResources queries modResource standard object fields. I suspect that your datestart and dateend are TVs, not resource fields. What you need is the
tvFilters property or a more complex query using a custom snippet.
With &tvFilters you could do this (untested):
&tvFilters=`datestart>=[[!today]]%,dateend>=[[!today]]%`
Also, I'd advise converting your dates to unix timestamps when comparing against another date. That way you're dealing with numbers instead of strings.