From my understanding of this problem you have the ANT+ ID for the speed/cadence device and you want to manually add it as the automatic paring tool does not find the device.
If this is correct then use the Add Device wizard, when you get to the last part of the wizard where you name the device you will see a Profile entry window. You will most likely have a five digit number followed by a h in this which matches the Heartrate monitor.
What you want to do is add a comma and then the five digit id of the speed/cadence sensor followed by the letter d.
So it should look something like: 12345h,67890d
Save that and it should search for these two id's next time.
The code letters are:
h = heartrate sensor
s = speed only sensor
c = cadence only sensor
d = speed/cadence sensor
p = power sensor
Hope this helps.