The employment data provided by LC are distinct values (not continuous). In other words, possible values are only 0,12,24,36,48,60,72,84,96 and 108. If you want to include all loans >=24, then just select all values 24 and greater in the select box.
However, if you wanted to create your own custom filter you could use the following in the customer filter input box: empLength >= 24. This would add the same logic as just selecting described above.
Hope that helps,