You can copy it onto the "Nokia N900/" folder...using File Manager.

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Another alternative way, easy way I learned is that you can do this....
Step1) Press the Power Button once...and select "Switch off!"
Step2) Connect to your PC USB port to your N900

Note: at this step your pc will detect your N900 and found as USB drive.

Step3) Now execute the "SetupSMM_Maemo_USA.exe" on your PC...now it select the internal drive,. (Depend on your pc...mine is Drive E: (my internal) and F: is my mircoSD)

This will be install directly to your internal drive.

Step4) after the program copy/sync program completed. you can disconnect your USB (I used the "safely disconnect from usb" on my pc).

Step5) Turn on your N900 again...after it is completely turn on, Using File Manager, run the SetupMobileMaps.deb file, (should be located at your "Nokia N900/" folder and complete your installation.

Step6) You should see a Sygic shortcut..run it...BINGO.

Hope this help