You need to be in USB mode for data signals. When selecting Digi mode on the Yaesu it gives you access to other menu adjustments such as processor, etc.
I have the settings for the FT-897D, but not the 450. I am sure such settings are freely available from a google search.
PSK Reporter is a good program to see if you are getting heard.
I set my transmit up by putting the radios readout into ALC mode, then adjusting the output from the sound card until the meter occasionally registered.
If you are not getting heard, then it could be that you have the output from the soundcard too low. You do, however, have to make sure you don't have this setting too high, or you will end up with lots of splatter.