I made a guess at the url based on how they format their live urls. I guess this could be one negative of having nice urls.
The current signup urls are:
Maximum: https://signup.37signals.com/basecamp/Max/signup/new
Premium: https://signup.37signals.com/basecamp/Premium/signup/new
Plus: https://signup.37signals.com/basecamp/Plus/signup/new
Old Plans:
Basic: https://signup.37signals.com/basecamp/Basic/signup/new