I have been trying to completely the "Garmin ANT Agent Authorization" thing and for some reason every time I try to authorize by typing in my "myGarmin" account info absolutely nothing happens.
I'm having the same problem with my 610. I can pair the device no problems but when it asks me to authorize my myGarmin username and password nothing happens, when I cancel I het a authorization key error.
I've followed Garmins instructions about the authorization key error (deleting the .bin file and then trying again) but get the same problem.
I'm wondering if it's a firewall problem, I'll try turning off the firewall and see what happens.
Did you ever resolve this issue, I'm experiencing the same thing. I have a MAC where I was hoping I could run two garmins (310XT and 910XT) to two accounts, however, they both sync to the same account, regardless of my efforts to remove devices, re - pair them etc. So I then tried deleting the 310XT from the MAC and installing it on an XP laptop. Everything seem to be going fine until the authorisation process where it refused to accept my login details even though they work fine for my Garmin connect account. I have Sophos antivirus running installed by work IT and I can't switch it off to test if that's responsible. All in, I've wasted several hours on this, written to Garmin several times and still unable to get it working, very frustrating indeed.