A few folks were having issues connecting their Freds to their amplifiers, so this seems like a good time to introduce Voltage Dividers--these will come in very handy for a lot of purposes as we continue.
The problem is, Fred puts out enough juice to power a speaker directly (approx 2 volts); your amp is expecting to see no more than line level (approx .2 volts at most--guitar or mic levels are even lower). So the signal is just too "hot" for the amp to handle, and it distorts or otherwise freaks out.
The answer is to attenuate the signal with a voltage divider, made from 2 resistors:
Basically, the signal from Fred gets soaked up by the 2 resistors in series; most of it gets soaked up by the 100k resistor, and about a tenth of it (uh... 10/110ths, actually, but who's counting) is left over for the 10k resistor... so if you put 2 volts in, you'd get about .2 out... which is much better as far as the amplifier is concerned. Changing the values of the resistors would change how much voltage comes out... if you used 2 50k resistors, it would divide the input voltage in half.
A Volume Control works about the same way:
The only difference is that as you turn the knob on the pot, you change the ratio between the two resistances (the resistance of the pot before the wiper, and after), so the amount of division changes from 0 to full on.
BTW, the schemo for the amp shows an optional volume control on the OUTPUT of the amp, which is a little unusual... it's not efficient, and it only works on a small amp. It's more normal is to put the volume control on the INPUT...