How did the Plaza Accord revalue the Japanese Yen?
The Plaza Accord was implemented through market mechanisms. According to Wikipedia Central banks of the signatory nations spent $10 billion dollars. They exchanged these dollars for other currencies. This increases the supply of dollars and may even decrease demand if so far the central banks have been purchasing dollars. Due to the higher supply and not higher demand the 'price' or exchange rate of the dollar fell. At the same time the countries also implemented policy changes. This would effect the mid to long term exchange rates.

The Japanese interest rate did indeed fall.  
http://www.tradingeconomics.com/japan/interest-rate
 Mind though that the US interest rate also fell  
http://www.tradingeconomics.com/united-states/interest-rate