Dooglus, you need to install the Ubuntu package I mention a few comments above...
Thanks. It took me a while to find the post you meant. It was this, in case anyone else needs it:
Ok, I installed Mono (and libmono-system-windows-forms4.0-cil which is needed to run casascius's app)
It works for me now, and seems pretty straightforward.
Here's how it works, with a worked example:
'be an escrow agent' creates an escrow invitation, which is in two parts A and B:
A: einvaT9CMj3qdZQZejm9DSse9c9MXNk5udVN2SQhTRJ6GKwJiX8wvukbK2fZ2ZkNo5LqmiMBfJNnLUDYoXYLo9XPa dvW38NbGo34LNLf9b
and
B: einvbT9CMj1gj74paurHWzgX4UeCpDgKUdiftFVdQxxYct23YMD2dAcfXd49dp61cpDRoK3sPbgKypXwR1h5ts9Bw CtWBJpUAvTvTvRabW
'be a payee' takes one of these codes (A) and creates a payment invitation (P):
P: einvpT9CMj791RwFxhxZMX37bTYXLabdw9sgM6iekpjjacUvDXf8rYMTYXeozySvW5tRUs6HwsyUuYbYuqm2xR66G YG6a9ArfDve4eQQsW
and an address:
14xEG5KnP1F5Y4ShstgSYT5hQeeqrWnMTp
'be a payer' takes the other escrow invitation code (B) and the payment invitation (P), and creates the same address:
14xEG5KnP1F5Y4ShstgSYT5hQeeqrWnMTp
After all that:
Escrow agent has A and B
Payee has A and P
Payer has B and P
Nobody has A and B and P
So all 3 parties have a different 2 of the 3 required pieces of information.
'collect your funds' takes all 3 codes (A, B, P) and creates both the address and its private key:
14xEG5KnP1F5Y4ShstgSYT5hQeeqrWnMTp
and
5JEiXmgXFYRxjPavisSaw5ipYAVpTjM5Vf7Xga33mrmyiBY834H
https://www.bitaddress.org/ has a 'wallet details' tab which confirms that that is the correct private key for the given address.