A PayPal donate button is completely separate to the Olimometer – the only link is that they might reference the same PayPal account. Each Olimometer can track the progress of a different PayPal account if required.
To create a PayPal donate button, log in to the relevant PayPal account and follow the instructions here:
https://www.paypal.com/us/cgi-bin/?cmd=_donate-intro-outside
You can then paste your donate button HTML code in to the Widget’s header or footer as I do on my personal blog. To do this, go to Appearance->Widgets in WordPress, drag an Olimometer widget to your sidebar, choose the Olimometer you wish to display, and paste the code in to header or footer fields.
If you aren’t using Widgets, then you’ll need to paste your button’s HTML code in to the WordPress HTML editor near the Olimometer shortcode as required.
If you wish to use multiple PayPal donate buttons directed to a single account but track them on separate Olimometers then this is currently not possible – you’ll need one PayPal account for each total you wish to track.