Unfortunately in Apsona it's not possible to filter records on long text or rich text fields. This is due to a Salesforce API limitation. Salesforce doesn't allow queries on these fields through it's API (Which Apsona uses to communicate to Salesforce) because it would be a big load on the system during a query so they just don't allow it. If you use Salesforce reports you can query against long text fields but it only searches through the first certain number of characters.
This is why you can't see fields like this in the filter options in Apsona. It is possible to display these fields in a report or Tab view, but you just can't filter on them.
One workaround is to apply all the other filters you need, run the report, export it, and do some searching in excel. Or try SF reports as mentioned above.