If you've found the AA website you'll probably know that the journey planner quotes 12.2 miles from Victoria Coach Station to Wembley Arena (44 minutes driving time) and 7.4 miles from Paddington to Wembley Arena (39 minutes driving time). I mention the driving times because I've found that �1-per-minute is a good rule of thumb for London taxi fares.
From Victoria Coach station, walk to Victoria Station (5 minutes). Take Victoria Line (northbound) for 1 stop to Green Park. Take Jubilee Line (northbound) for 11 stops to Wembley Park. 8 minute walk to Wembley Arena.
From Paddington, take Circle Line (eastbound) 2 stops to Baker Street. Take Jubilee Line (northbound) 9 stops to Wembley Park. 8 minute walk to Wembley Arena
Alternative route from Paddington: Take Bakerloo Line (southbound) 2 stops to Marylebone. Take Chiltern Rail service (just 9 minutes) to Wembley Stadium station. 4 minute walk to Wembley Arena.
NB: The redevelopment of Wembley Stadium is linked to station renovations. You should check your journeys before travel. Phone Transport for London Travel Information (24 hours) on 020 7222 1234 or use the journey planner here:
http://journeyplanner.tfl.gov.uk/user/XSLT_TRIP_REQUEST2 ?language=en
The Wembley Arena Pavilion website has directions from the local stations to the arena:
http://www.whatsonwembley.com/htdocs/visitors_guide.php? name=visitor's+guide
Chris