Seldén type number 502-520-02 is a three sheave foot, I think you ordered the wrong type. The 5 sheave foot is type number 502-520-03. Maybe the easiest solution is to return the part to your chandler and order the other type. The number Trident uses (502-511-02) I am unable to find on the Seldén pages. Note that number 502-511 is a series 3 adapter, what ever that is.
If you go to the Seldén web pages you will see some sheaves listed. Maybe a three sheave foot can be converted in to a 5 sheave foot but the web site does not say what sheaves you need for that, nor does it list the axle and possibly you need two clips to fix the axle in place. See for yourself, here: http://www.seldenmast.co.uk/fittings/listing.asp?Category_ID=16
If I were you, I would ask the question directly to Seldén via: email@example.com
Note that the sheaves shown on the Seldén pages are also offered by Trident. See: http://www.tridentuk.com/sailing/Wayfarer_Spar_Fittings.html. Scroll to the bottom of the page.