If it is anything like the 2006 Sienna, it is probably in the fuse box under the driver's side of the console. It is difficult to see, but if you lie on the floor under the steering wheel you can see the fuse box and it is easy to pop the cover, which has a legend telling you what the fuses are.
I just changed the fuse on my 2004 Toyota Sienna. I had similar problem. The 12 volt power outlets stopped working. It was the fuse. As noted above, it is located on the driver's side of the console, under the steering wheel, to the left. You need to remove the small black cover and there is a legend underneath. Ours was a 15 amp fuse, 3rd one down, abbreviated as "power outlet." Hope this helps.
The 2003 Toyota Sienna is 6 ft. 1.4 in. (73.4 in.)12V front and 12V rear power outlet(s) wide.
The 2001 Toyota Sienna is 6 ft. 1.4 in. (73.4 in.)12V front and 12V rear power outlet(s) wide.
The 2002 Toyota Sienna is 6 ft. 1.4 in. (73.4 in.)12V front and 12V rear power outlet(s) wide.
The 2011 Toyota Sienna is 6 ft. 6.1 in. (78.1 in.)12V front and 12V cargo area power outlet(s) wide.
The 2007 Toyota Sienna is 6 ft. 5.4 in. (77.4 in.)12V front and 12V cargo area power outlet(s) wide.
The 2012 Toyota Sienna is 6 ft. 6.1 in. (78.1 in.)12V front and 12V cargo area power outlet(s) wide.
The 2010 Toyota Sienna is 6 ft. 5.4 in. (77.4 in.)12V front and 12V cargo area power outlet(s) wide.
The 2009 Toyota Sienna is 6 ft. 5.4 in. (77.4 in.)12V front and 12V cargo area power outlet(s) wide.
The 2005 Toyota Sienna is 6 ft. 5.4 in. (77.4 in.)12V front and 12V cargo area power outlet(s) wide.
The 2008 Toyota Sienna is 6 ft. 5.4 in. (77.4 in.)12V front and 12V cargo area power outlet(s) wide.
The 2014 Toyota Sienna is 6 ft. 6.1 in. (78.1 in.)12V front and 12V cargo area power outlet(s) wide.
The 2013 Toyota Sienna is 6 ft. 6.1 in. (78.1 in.)12V front and 12V cargo area power outlet(s) wide.