>> Can anybody explain me how do i have to determine these support conditions. Especially this rotation part.
Well basically, the supports are fixed to nodes in your structure. that is, each node might have a support. ("Lagerung".). Each support covers 6 degrees of freedom. for each direction, it can be either completely fix ("Fest") or it can be defined using a modulus of resilience. (which, in case of xrst, is "fixed" at a value of 1e10 and "free" with the value of 0.)
When the program prompts a message about the structure beeing instable, in most cases this means there are missing supports for at least on direction. (for example, incase there is no support for the z-direction, the structure will obviously be instable in that direction.)
i'm not sure what you mean by "this rotation part", as there are not rotatable supports in xrst. (in "baustatik", which is actually the newer version of out 3d structure-frame software, rotated supports are supported.).
Hope this helps,