Editing assembly on Mac OS X

I'm currently disassembling a Mach-O executable(the executable runs on Mac OS X, to be specific) in IDA Pro on windows. I have one instruction which I want to change, but it doesn't seem that IDA Pro allows you to do this. So, I tried using otool on OS X to dump the assembly(wh开发者_如何学JAVAich it does fine). However, after editing the assembly in otool, I tried to reassemble it using as <file name of modified otool -tV output>. This greets me with a bunch of errors like Junk character 43 (+). What's the easiest way to change just a single instruction in a Mach-O binary on Mac OS X?

I've used both otool and otx to disassemble various binaries, and I've found the easiest way to edit them is to figure out what the assembly of the new code I want is (using nasm or similar), then just use a hex editor (I prefer Hex Fiend) on the binary file.

I use Synalize It!.

You can load also a custom grammar (example for png files, showing header, tags, etc...)