(And getting back.)
Lake Tornow is located about 60 km east of central Berlin (e.g. Kreuzberg), half way to the border with Poland. (Here is a map link.)
The address for GPS devices is Am Tornowsee 1D, 15377 Oberbarnim. You will find detailed directions in German on the Haus Tornow web site.
Please note that it is a relatively remote location. Using public transportation requires some planning, and will only take you as far as Buckow, a pleasant 6 km hike away. From Berlin Lichtenberg station, take a regional train to Müncheberg, and then a bus or the vintage Buckower Kleinbahn train to Buckow. We have compiled all the relevant schedules in a pdf document, and will ask those with cars to pick up non-motorized guests in Buckow on Friday night and again on Saturday morning. Please note that “our” Buckow has nothing to do with the Berlin district of the same name.
(There is also a bus that stops at Pritzhagen, 1.2 km north of the lake, but it only runs three times a day on Saturday and Sunday, and there is no service on Friday). You could also take the S-Bahn to Strausberg, but it is almost half an hour’s drive from Tornow, and taxis cost approximately 45 euros one way.
To find cheap flights to Berlin, we recommend using a travel search engine such as momondo.com or kayak.com. Book early to get the cheapest deals. The same goes for train travel: tickets at bahn.de can be booked up to three months in advance; booking early usually gets you a very good deal.
Getting back to Berlin on Sunday is pretty straightforward, and we will post a bus/train schedule in Haus Tornow. For those not staying at the house who want to leave after (or during) the party on Saturday night, the good news is that the S-Bahn from Strausberg to central Berlin runs all night (e.g. at 23.46, 0.06, 0.26, 0.46, 1.23, 2.23, and 3.23). We can order one or more large taxis from Tornow to Strausberg, which should be affordable if you split the cost.