I must admit that I've never used the I/O Assistant. I just go ahead and use the lower level VISA functions. It sounds like you might have problems with the configuration of the serial port. It might be a baud rate thing or incorrect termination character. You might want to try some of the VISA serial examples that ship with LabVIEW or convert the Express VI to a standard VI. I'm assuming that you've already tried the commands in something like Hyperterminal and everything is okay there.
To use the C code examples, you'll need a C compiler like MS Visual C and create a DLL (easier than a CIN, imho). If you go to the help menu and select Search the LabVIEW Boookshelf, the pdf file that opens will have a link to "Using External Code in LabVIEW" that will explain how to create the DLL and call it from LabVIEW. However, I think it'll be easier to get the VISA functions to work correctly.
