Every font can contain at least 256 characters. Your signature is stored in a single character location (we normally store it in upper and lower case) and all other locations are blank. Blank locations display and print a box. Verify that you have selected the correct character in your font and the correct case. Check that your CAPS LOCK is on or off depending upon whether you wanted your signature or logo to be accessible from the lowercase or uppercase version of the key. If no key was specified, your signature or logo will be found under <a> or <A>. If you are a Windows user, you can also use the "insert symbol" feature available in most applications to display the contents of your font and ensure you choose the correct character.
If you have just finished installing the font, it may be necessary for you to re-boot your system so that certain applications are aware of the new fonts.
Although rare, it could be that the design created for you is too complex for your system to handle. If your logo or signature appear on screen but do not print out (whereas other smaller elements from the font do), then there is clearly an overflow problem and you either need to get a more powerful computer/printer or have us break your signature or logo down into more manageable pieces.
Occasionally, a signature may display as a box, but print correctly. Changing point size, up or down, fixes this anomaly. This problem is less pronounced in Windows 95/98/2000/NT than Windows 3.X and also varies on MAC depending upon the OS version.