I just purchased a Burkert 6011A from ebay and wanted to be sure I wire it correctly.
The 6011A is 24V DC, 4W.
To power it I found, also on ebay, an AC adapter that outputs 24V DC and 400mA. I just want to be sure that the amperage is enough.
Also, as far as I've read it's just a matter of splicing the solenoid wires onto the plug and I'm good to go, correct?
As mentioned by Seattle_Aquarist, your adapter is fine.
To answer your second question, just take the solenoid wires, strip the insulation off the ends of the wire, do the same for the adapter, and then solder together (you can insulate with heat shrink tubing).
You can be a little more fancy and get a DIN housing for the solenoid as well, but it is not necessary.