When using an attenuator with a combo amp, place the unit between the amp's speaker out and the speaker itself.

Most combo amps have internal speakers that can be unplugged from the amp chassis using a 1/4" phone plug.  UNPLUG the speaker from the amp.  Run a short speaker cable from the output of the amp to the INPUT of the attenuator.  You may then plug the speaker directly into the SPEAKER OUTPUT of the attenuator, or use a male-to-female extension cable if the speaker's wire won't reach.

You should use speaker cable only, and short cables work best to prevent signal loss. 

FAQ, Attenuator Terms and Descriptions
BypassThis switch bypasses the attenuator entirely, as if it is not connected between the amp and speakers. It is the same as a true bypass on an effects pedal.
Range Switch This is a fixed amount of attenuation in addition to the setting of the main speaker volume (attenuation) control. It has the effect of extending the range of the main speaker control. The fixed amount of attenuation depends on the impedance of the amp and speaker, but is typically 6db.
Why are there two speaker output jacks? The two speaker output jacks are in parallel. The actual speaker impedance isn't important. You can connect any impedance to the output of any Weber attenuator.
Will the unit get hot? There will be some heating because we use a combination of power resistors, wirewound rheostats, and a speaker motor in the attenuators.
Where should I put it in the rack? I would either locate the attenuator in the top position of the rack, or in a lower position that has an empty space above it.
What kinds of cables should I use? You need to use speaker cable only. I prefer at least 16ga wire with very good quality 1/4" phone plugs on the ends. I would not go over 4ft for the cable length. If you get feedback or squealing from the speakers, you may need to relocate the attenuator and use shorter cables.
What level is the line out signal? The line out signal is typically 1/10 (-20db) the level of the amp's speaker output signal level.
Why is the main attenuator control hard to turn? That is a high power wirewound rheostat because it has to handle the full power of the amp. So, it is grainy feeling as you turn it. It will be fine, though. It is made that way.