Hey Scott,
You either need a multiplication symbol or a space between the expression and the constant you multiply the expression with. In the code below I inserted spaces.
Manipulate[
Plot[{Sin[2 Pi/a x], c Cos[2 Pi/b x]}, {x, 0, 4 Pi},
PlotStyle -> {GrayLevel[0], GrayLevel[.5]},
PlotRange -> {-4 Pi/2, 4 Pi/2},
AspectRatio -> 1], {{a, 2 Pi, "Period for Sine"}, .1,
4}, {{b, 2 Pi, "Period for Cosine"}, .1,
5}, {{c, 2 Pi, "Amplitude for Cosine"}, .1, 5}]
Hope it helps.
