Well, you've not given much to go on so let's start with the fundamentals:
1- Make sure Office is fully updated (12.2.4)
2- Make sure OS X is fully updated (10.6.3)
3- Make sure your printer driver is fully updated (check the mfr's site)
4- If you're just looking to print a single envelope as a document be sure
to click the Page Setup button in the Envelope dialog so you can confirm the
orientation your printer needs to print the envelope correctly. Printing is
not Word's job -- that's handled by the OS & your printer software.
If that isn't enough to get it right you'll have to provide more explicit
descriptive details about exactly what steps you are taking & what the
specific results are... Including what your definition of "sidewise" is.
HTH |:>)
Bob Jones
[MVP] Office:Mac