Turns out the key is given in Appendix 1.1 of the draft for "JSON Web Signature (JWS)" in the JSON Web Key format as:
{
"kty":"oct",
"k":"AyM1SysPpbyDfgZld3umj1qzKObwVMkoqQ-EstJQLr_T-1qS0gZH75aKtMN3Yj0iPS4hcgUuTwjAzZr1Z9CAow"
}
I misunderstood that the two links named "Appendix A.1" at the end of the "JWT Example" section actually go to different locations. Tha passage with the links is reproduced below:
This computation is illustrated in more detail in Appendix A.1 of [JWS]. See Appendix A.1 for an example of an encrypted JWT.