Switch to the code view in Frontpage (so you're looking at the code, rather
than the graphical user interface view). Then paste the paypal code into
your code at the spot you want it.
If you put the "Split view" on (you get the normal and code view, so you can
see the code and design view at the same time.
Position your cursor in the Design view at the point you want the code, then
click inside the code view window; this moves the cursor to the equivalent
point in the code, as it is in the design view. Then copy/paste the paypal
code into the code view window at the point where the cursor is flashing,
then click back in the design view which refreshes with the newly added
element (your paypal button(s) and the form to make them work etc.