I set a hosts condition in an ACL to do a lookup in a file of IP addresses, but it doesn't work.

Did you remember to put net- at the start of the the search type? If you set something like this:

accept hosts = lsearch;/some/file

Exim searches the file for the host name, not the IP address. You need to set

accept hosts = net-lsearch;/some/file

to make it use the IP address as the key to the lookup.