From the "Target" menu. select "Reports", then scroll through the options in the drop-down box.
If all you need to find is the next available address of any memory type, when entering ladder logic, and prompted for an address or variable, simply type in the memory type prefix without any specific address, (for example, %I) and Proficy will select the next highest available address. It will not fill in unused gaps, it only checks for the highest address in use.
Another possibility is to go to the variables list and sort by address. You can also filter for only the memory type you're interested in.