When you go to download the map by default it tries to download it to the satnav i.e. not the SD card. When the website recognises the satnav it shows it in a drop box with a drop-down arrow. Click the drop-down arrow and it should give you the option to download the map to your computer. Download it to the hard drive.
After it's downloaded it may begin the install automatically. If not click the START button on your computer and go to GARMIN. In the garmin menu click MAPINSTALL to begin the installation.
When the install program finds your satnav and you get the option to install the map there again click on the drop-down arrow and select the drive letter of the SD card. This will be different to the drive letter for your satnav. The map will then be installed to your SD card.
Before you begin to install to the SD card you should know that it can be very hard to get get the 200W, and other satnavs of the same age, to run the map from SD cards. It will install to it without problem but when you try to operate the satnav then it cannot load the map and will close down again unless you remove the SD card. I recently had this problem with a garmin nuvi 760 and contacted garmin about it. They told me the problem is that 4 Gb SD cards are SDHC which operate differently from SD. They said to get a class 2 SDHC card. (You can identify the class by the small figure on the card. It will be either 2, 4, 6, 8, or 10). Before you install the map format the card. Also use webinstaller to check that you have the latest version of operating software on your satnav.
Format the SD card with "Panasonic SD Formatter" available from http://panasonic.jp/support/global/cs/sd/download/sd_formatter20.html
rather than the standard windows format function. Select the fast format option and be sure you have selected the drive letter of your SDHC card as it will format your hard drive just as easily if you select it by accident or your satnav.
To update your software you can download webupdater from http://www8.garmin.com/support/collection.jsp?product=999-99999-27
Connect your satnav to the computer and run the program. It will search for any updates to your software.
Class 2 SDHC cards are available on ebay if you can't find one in the shops.
After failing to get mine to work on a class 4 SDHC 4Gb card I followed their instructions and it worked for me. Hope it works for you too.