Enable extras-devel in your N900's application manager.
Go to the Enable Repositories Automatically section of this site:
http://maemocentral.com/2009/12/29/h...ith-one-click/

Once that updates, click on Download inside your application manager and scroll down the list of applications until you see Picodrive. Click on it.

Please be warned that extras-devel software is still in development and most of the time can be tricky and hazardous for users new to Maemo devices.