Unfortunately, with the exception of the addition operator, you can't automatically break lines in Mathcad 15 (and below).
There are several methods of getting round the problem, which usually take up more vertical space (I assume that's less of a problem?).
The first thing you can try is reducing the size of your standard font - that may squeeze a few expressions into the page,
The second thing (which I assume you've either done or don't want) is to use Landscape mode.
Then you enter the realm of modifying your expressions to use less horizontal room. For example, ..
Put your function arguments into a vector and either re-assign to the variables within the function or modify your function to pick the appropriate elements from the vector.
Break up long expressions manually and piece them back together again on separate lines.