It seems that Mango M2M and Mango M2M2 are already retired and with no stable version and also no sources, so You'd better using Scadabr, it is a fork from Mango.
Scadabr's forum is only in portuguese but you can post english messages there.
Their suport is not good as the Mango used to be but there are sources available and it seems to work fine. There are lot of bugs also but the team is still active.

http://sourceforge.net/projects/scadabr

Also take a look at this:

http://sourceforge.net/projects/scadabr/files/Software/Docs/OPC Configuration ScadaBR.pdf/download

and this

http://mango.serotoninsoftware.com/forum/posts/list/15/599.page

Dear,

I want to use the new Mango M2M2 in my home automation system in order to "talk" with my Siemens S7-226 PLC. Since I did not found a direct integration, I've installed the Siemens OPC server component. Unfortunately, I do not succeed in configuring the OPC server in Mango M2M2.

Whenever I want to register it as datasource, I enter all data (username, password, servername, ...) but I can not select the OPC server installed on that server from the listbox. Probably I am doing something wrong overhere.... can somebody help me out please?

I am using the latest M2M2 binary (found on this website) together with the OPC DA module (version 1, I belive).

Kind REgards,

D