Check to see if you have the license, I believe now is under advanced rules.
Help > Installed Options [x] Advanced Rules
SIgnal pairs can ONLY be routed as pairs in the PADS Router, and then only if you have the "high speed" option in your license. (this is included in the ES suite)
There are a trick you can use to route diff pairs in Layout: For the purpose of routing only, increase the trace width of the trace pair by the value of the pair 'gap'. Route the traces exactly edge to edge. Once the route is complete, change the trace width back to its nominal value.
Granted this is only practical for board where only a few pairs exist. But it is an effective tool for doing pairs where the capability does not exist.
How can we set the phase tolerance for a differential pair in the Pads Layout 2007 tool?
And also how can we set the rule to match the length of the pair?