The sensor you've shown is the LiPO voltage sensor and it plugs into the sensor hub. So the answer is NO ... it won't plug directly into the Rx. You'd need the sensor hub which then plugs into the data port on the Rx (NOT A1/A2).
For measuring your flight battery pack, your options are:
1. Battery voltage sensor - http://www.frsky-rc.com/ShowProducts.asp?id=41
(product ID - FBVS-01)
This requires you to solder one end of the sensor to the battery leads (either the +/- leads on the battery side of the ESC or the +/- leads from the balance leads). You can also choose whether you want to measure 5S packs (the default setting on the little board) or solder it so that it measures 3s (or 1s) packs. This is a very simple solder job and is explained in the doco. The reason to do this is that if you're flying 3S packs, you'll get greater resolution on the voltage being sensed by making the change.
Then you plug the other end of the sensor directly into the A2 port of the Rx.
Note that the built in sensor in the Rx measures your Rx voltage only - not your flight pack voltage - so it will be showing 5.0, 5.5 or 6.0 volts depending on how you've configured it. This (IMHO) has limited value if you're also measuring flight pack voltages. The Rx voltage will be shown as the A1 measured value.
2. LiPO battery sensor - http://www.frsky-rc.com/ShowProducts.asp?id=111
(Product ID - FLVS-01)
This is the cell by cell sensor with teh OLED display. In order to send this data back to the Tx, you need to include the sensor hub (http://www.frsky-rc.com/ShowProducts.asp?id=92
). This sensor is connected to teh flight packs balance leads and then connects to the hub. The hub is then connected to teh data port of your Rx.
Note that if you're using the FLD-02 display like i am, then only A1, A2 and RSSI alarms can be set on the DJT/DHT Tx modules. (ie you can't set an alarm to sound for the FLVS-01).
If you're using the DHT-U display/Tx combined unit, then i understand that this allows you to set more alarms (including those of the FLVS-01). Hopefully a SW upgrade will be available soon so that FLD-02 users can do this too ... but no word on that yet.
Personally, i have both the FBVS-01 and the FLVS-01+hub. I'm only using the FBVS-01 at the moment and setting my alarms using the FLD-02 for low voltage on my flight packs.
Hope that helps.