Where in version 1.6 or 2.0 can I check the IP address of a given host?

Try change column vield from yours “host addresses (IPv4/IPv6)” for example “Host adress (Primary)” in the ‘edit view’. Mayby you use only ip addresses in nagios configs files, then resolve dns cache is empty.
And if you many users in check_mk set this view globally, set mark “Make this view available for other users” in “General Properties”.