First: you can't send symbols to a synth control. You can only send numbers.
Second: your example doesn't say what freq should be if the test is false. In fact, you should write it in more of a dataflow style such as:
freq = if(testcondition, 200, 300);
That's the kind of thing that will work in a synthdef.
Third is a slightly frustrating thing in sc language, which is that the ==
message is always evaluated at synthdef compile time - the equality is checked once, and then never again. In order to have "live" equality checking, you can use this slightly clunky expression:
BinaryOpUGen("==", thinga, thingb)
So in summary you might write
freq = if(BinaryOpUGen("==", myParam, 1), 200, 300);