Great question. I think I understand what you are asking.

First, select the report you need, then under the "Filter criteria" section, click the "Filter" button.

Then, under the "Linked Entities" tab, click the client search button at the right.

With the Advanced Search enabled, click the "Custom details" tab. Here you can select the custom fields you have created for your clients and use them as filter criteria in your reports.

I hope this is what you are after. Let us know how it goes!

Scott