I am having the same problem, in the past the PMDG Operations Centre has actually updated my products but now it click the ok button after selecting 'Install Update'. It does its sequence then just has a bar loading with Sending Request and nothing happens.
The main issue is i bought the 600/700 737 last night and it wont run because i can't update the 737-800/900 addon, also that addon i bought so long ago is a BOXED addon so i am stuck :(
If somebody could offer me any advise or has had the same issue and resolved it, id be so grateful for some help :)