Usually, you would expect iPhone/Android to find your printer right away - after all it's standing right here. However, in some cases it is a bit more complicated than that, and you need to complete a few extra steps.
Here is the setup you are looking for:
Step 1: Prepare Desktop computer
One of your computers must be already connected to the printer. You will use it to receive and print documents sent from the phone.
Step 2: Install PrinterShare client
Download and install PrinterShare client for the desktop computer.
Step 3: Share the printer
In PrinterShare client on desktop computer, find your printer in "Local Printers" list and click "Share".
Step 4: Find the printer on the phone
Locate your user id (displayed at the top of desktop client window).
Enter user id as a search string on your phone. To get to the search screen:
iPhone client 1.3: on main screen, tap "Settings", then "Printing Preferences". If you already have a printer selected, tap the printer.
Android client: press Menu, then tap "Remote Printers".
Now, after search, you should receive the printer you just shared. If you can't find it, try to uncheck "online printers only" in search parameters.
It is also a good idea to fill in your profile on desktop computer (select User | Profile from main menu). This way the phone will show you some details along with the found printer, and you will not add someone else's printer by mistake.
Step 5: Print
Now you are ready to print. But your document is not going directly to printer. Desktop client will receive it and notify you. Then you have to open Incoming Documents window, select the new job and click Print.
This step is necessary to prevent someone else from using your printer. You can change the way desktop client handles documents from your phone by setting an automated action (select Printer | Automation from main menu).
See Privacy for suggestions on further improving your printer's security.