The 2521 was intended for a vending application and doesn't have the "always enable" function on the switches. That said you can build / buy an enable plug.
Mars Part Number: 250074011
https://www.dsvendinginc.com/p-2360-d250074011-marsmei-harness-vn-series-125-enable-jumper.aspx

Basically it ties the $1, $2 and $5 high enable lines to the out of service not line and the $1, $2 and $5 low lines to ground. Or find and AE2400 series which has the always enable function.

Pinout here: http://www.unitedchangers.com/MEI%20Series%202000%20%20Operational%20Service%20Guide.pdf

Oh and you can use a VFM1 (which would only take $1) or a VFM3 ($1,$2 and $5) but honestly I wouldn't waste any money on them. The VFM3 hasn't been updated in years and won't take most of the newer $5's.

If you set the validator to 4 pulses per dollar it will send 20 pulses for a $5 bill. The number of credits established will be based on the machine setting for price per play (so yes if you are set for 50c a play you will get 10 credits). For a 110v validator you should have 110v ac across pins 4 and 6 (on the 9 pin molex) and your credit contacts will be on pins 7 and 8.
Basically the validator is pretending to be a coin acceptor with the pulses representing the micro switch on the bottom of the coin acceptor closing.

Any other questions feel free to ask. If you post a picture of the front of the machine I can tell you what model you need. Most likely it's an AE2411-U5E