I just ripped this all off the Wiki:
http://wiki.ffxiclopedia.org/wiki/Macro~~~
Storing Configuration Data on PlayOnline Servers
It is possible to store configuration data (macros, configuration settings, and map markers) on PlayOnline's servers, in addition to your game machine's own hard disk drive. You can store the configuration data for one character's worth of data on the PlayOnline servers.
Saving to the Server
At the character selection screen, highlight the character to save the data for, and press the following buttons/keys. Then choose the slot in which you want to save the configuration information for the highlighted character.
PlayStation 2: L1 button + L3 button
XBOX 360: LB + left stick button
PC/Keyboard: Shift + Alt + Ctrl + B
Restoring from the Server
At the character selection screen, highlight the character to restore the data for, and press the following buttons/keys. Then choose the slot from which you want to load the configuration information for the highlighted character.
PlayStation 2: R1 button + R3 button
XBOX 360: RB + right stick button
PC/Keyboard: Shift + Alt + Ctrl + R
Deleting Data and Restoring Default Settings
At the character selection screen, highlight the character to restore the data for, and press the following buttons/keys. Confirm deletion to restore default settings for the currently highlighted character.
PlayStation 2: R1 button + L2 button + R3 button + L3 button
XBOX 360: R2 + L2 + right stick button + left stick button
PC/Keyboard: Shift + Alt + Ctrl + D
Backing up Macros
PC client users can back up macro files while not logged on to the character. The location for files containing the macros for PC clients is at:
C:\Program Files\PlayOnline\SquareEnix\FINAL FANTASY XI\USER\{ID}\mcr*.dat
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If you use Windower and have scripts in your macros, I'd advise against uploading those to SE's servers. That may just be me being paranoid. Also, I guess you can just copy your macros stored in the aforementioned folder and simply paste them into the same folder on your other PCs.