On that page it says that you can donate through the Charities Aid Foundation website and it provides a link to their homepage (
http://www.cafonline.org/). Unfortunately the CAF site is an abomination of ASP.Net and there is no way for the V&A or me to link to the specific charity page (or for Google to index it); you have to search for "Victoria and Albert museum" and click on the JavaScript link to get to a non-bookmarkable page giving information on "Friends of the V&A" (which is the charity that deals with memberships and donations for the V&A), Hit the donate button and it asks how you want to pay - including credit cards.