The limiter has a fixed threshold of +20dBu. It won't become active unless the output level reaches this (its purpose is to prevent overload of the optional A/D converter, so the 'active' LED should not be lighting under normal recording situations). The limiter comes after the output level control in the signal chain.
To test whether the limiter is working properly, you need to measure the output of the limiter by looking at the INT LED meter. Make sure the 'bypass' button is pressed in so that no processing on the unit is active. Now when the limiter is engaged, and you set the input gain and output fader so that the output signal level goes above +20dBu, you should see the 'Active' LED come on and the LED meter reaching 0 but not 'O/L'. Then, if you disengage the limiter, the 'Active' LED will turn off and the 'O/L' LED will light on the LED meter. If this is not happening, then ensure that the output level is high enough for the limiter to become active.