OK, so the speaker is an 8ohm load and the amp putting out up to 12 watts. We don't know if this is peak watts or RMS, for now I'll work it out based on it being RMS (the meter may under read if the 12 watts is measured as peak, which is safer for the meter). Ideally you want to see the meter jumping about without having the amp up full, so some experimentation of values might be needed (or add an extra variable resistor perhaps).
First to workout the current flowing through the speaker at 12 watts/8 ohms which is 1.22 amps (Watts = current squared x resistance). This also means that there will be 9.8 volts across the speaker (Watts = voltage x current).
(Use this link if you want to confirm the calculations: http://www.csgnetwork.com/ohmslaw2.html
The specs show that the meter needs 500 uamps for full scale deflection which is 0.0005 amps. So with 9.8 volts across the speaker we will need a resistor of 19,600 ohms to give 500 uamps (0.0005 amps). We will also need to add a diode since the meter measures DC and we're monitoring an AC signal.
You need to connect the meter in parrallel with the speaker, but with the 19,600 ohm resistor (or nearest available value since this isn't a standard resistance) and diode in series with the meter to protect it from excessive current (the diode protects it from reverse current).
I need to draw something out to show how you connect it all up and add it to this thread, which I will do later on (I have some jobs I should be getting on with