Originally, free TeX previewers were only capable of displaying bitmap (PK) fonts, but free Type 1 font rendering software has been available for some time, and many previewers now use such facilities.
The alternative, for previewers, is automatic generation of the
requisite PK files (using
gsftopk, or similar,
behind the scenes).
In the unlikely event that your previewer isn't capable of either, you have a couple options:
gsftopk(designed for use with the ghostscript fonts) to make PK bitmap fonts which your previewer will understand (a process similar to the way some browsers fo the job “automatically”) This can produce adequate results, also suitable for printing with non-PostScript devices. (Note: if you purchased the fonts, it is advisable to check that their licence permits you to convert them, for private use, in this way.)