Easy! :)

Windows:
(only Win2000/XP/Vista/7 supported - earlier versions have limited OpenType and Unicode support)
1) Unpack the zip file (using the built-in unzip function of Windows, WinZip or any other unpacking utility)
2) Put the font file (ends with .otf or .ttf) into C:\Windows\Fonts
2b) Easy option for Windows Vista/7 : Right-click on the font files > "Install"
3) Quit and restart any programs you had open (no need to restart your machine)

Mac:
(only MacOS X/10 supported - earlier versions have no OpenType and Unicode support)
1) Unpack the zip file (using the built-in unzip function of MacOSX, StuffIt Expander or any other unpacking utility)
2) Put the font file (ends with .otf or .ttf) into home/Library/Fonts
2b) Easy option: double-click the font file, then click "Install font" button of FontBook.
3) Quit and restart any programs you had open (no need to restart your machine)

Unix etc.
I have no experience with other operating systems, so you're on your own here - but if you have installed and use another operating system you're probably technically savvy enough to know how to install fonts ;)

As long as you have installed them correctly our fonts are shown alphabetically along your other fonts.
Except:
If you use older Adobe CS programs on Mac you may find them at the bottom of the font menu - this is because the system (correctly) flags the fonts as multilingual, and so the CS programs sort them at the bottom part of the menu (where all non-western fonts are gathered). As all our fonts also contain the normal western character set they should not be sorted out like this - this is a "bug" that Adobe was aware of, and it has been fixed in recent editions of the CS programs...

All our fonts are infinitely scalable; the limitations are in your software. A common problem is scaling fonts in Microsoft Word. If you choose Format / Font you can type in any number you like under "size".

At the time of this writing (Feb 2008) you can’t. After installing one of our fonts, you may be able to select it in your email application but the person at the other end won’t see that same thing unless they have the same font installed.
If you really want to use our fonts in email applications, make sure the people at the other end have the same fonts installed (but they will need to purchase their own license).

The easiest solution to use our fonts (at least for headings or titles) is to use image creation software, save the images as GIF/JPG/PNG files and place them on your web page. Easy, oldfashioned and foolproof! ;)

If you’re creating a web page using Flash, you may embed our fonts in those files. Consult your Flash manual. You may also use the fonts with (the Flash based) sIFR routine.

If you’re using PDFs, make sure the fonts are embedded in a secure way. Consult your Acrobat manual.

For anything else there are limitations and special licenses needed - check our page on @font-face embedding.