It ought to work out of the box however you can try a couple of changes in your profile under printing. Assuming you are using the default profile:
Under printing, first try turning on the "print while transferring" checkbox. After making this change you need to stop and restart telnetpro. (use the picture of the printer on the toolbar for a "quick" printscreen function).
Next, if that doesn't work, in the printing profile section again, change the
print method. The default is "normal" you can try "direct" if the printer is connected to a parallel port in which case the device is LPT1: Otherwise try "raster" and reselect the printer in the printer selection box at the top of that dialog.
Let us know which method works for you...