IPAM Express Network discovery

I have installed the IPAM Express Freeware. I cannot make it do a network discovery of anything on my network.
My IP addresses are 10.250.140.x 141.x 142.x and 143.x The sub-net mask is 255.255.252.0
When I try to add a network I get different errors:
If I try to add a network under IPv4 with the subnet 10.250.140.1 and the sub-net mask of 255.255.252.0 it tells me "network address does not belong to the CIDR 22.
If I try to add a network under IPv4 with the subnet 10.250.140.1 and the sub-net mask of 255.255.255.0 it tells me "network address does not belong to the CIDR 24.
If I add it with the sub-net mask of 255.255.255.255, it is added but nothing is discovered.
I am at a loss as to how to use this software.
Description sounds really easy, but I cannot seem to get it to work.
Thanks.
Mark.