The explorer pass is great value for money and has the added advantage of allowing you to chose from a really wide selection of tours and attractions. We went for the 3 version and went on the Hop-on-hop-off downtown tour (highly recommended), Madame Tussauds (it was raining!) and Top of the Rock (widely regarded as a better - and less busy - option than the Empire State). We were able to skip the queues in both TotR and MTs which was great (but did peeve a couple of people in the other queues ;-) ).
You just have to take your voucher from Viator into one of three locations to pick up your card(s) - we used the NBC location in the Rockerfeller Center - just go to the Studio Tour desk on the top floor of the NBC Experience shop.
In summary - a highly recommended, flexible and great value option.
Overall the explorer pass was great, the pass was easy to get and there were plenty of options to choose from. The night schooner sail was amazing, really a MUST do. The staff was all great and very friendly and the complimentary beverage was definitely much appreciated. Unfortunately I would not recommend the uptown city sights hop on hop off tour. The staff was very uninformed for most of the tour reading off the names of the stores in the area. They didn't seem to know any actual facts about the area. We also had to wait a long time in line for the bus to have the first one too full to get on, the second one the driver got off without a word and started smoking not telling us he was on a break as we continued to wait. Finally the third one, the one we actually got on we got yelled at by the bus driver for not being in line and even threatened not to let one passenger get on. What made it worse was that were not in line because every time a bus came they parked it in a different place meaning the whole group walked up and down the block chasing after the buses. I would definitely avoid "city line buses" in the future. Having said that the schooner sail really did make up for it.
DO NOT purchase this pass if you are limited in time ... it was a game to redeem the coupons and secure the pass (go to Grand Central, walk 8 blocks to redemption center for pass, walk back 7 blocks to secure tour bus pass, walk back 6 blocks to board bus) ... this was the scenario throughout the day ... bottom line: we were unable to utilize most of the features due to lack of time .. the two features we did utilize briefly were great - be warned to prepare to walk to multiple locations to have your pass 'enabled' to secure the features and only purchase if you have 3+ days in NYC available ... we encountered many other people with similar disappointments (we would've saved 50% if we had just gone to and paid the attractions individually).