Although there are more complex/recommended ways of enabling country stats in awstats (the geoip plugin), a very quick and easy way to enable basic country stats in awstats is to edit:
/home/username/domains/domain.com/awstats/.data/awstats.domain.com.conf
and set:
DNSLookup=1
Note:
That this will likely slow down the computation of stats significantly since many reverse IP lookups would need to be done.