Ok. You need to break down the basics of the psi calculation into two parts. In this case the psi value is the difference between the total heat loss through your corner junction and the heat loss through sections of wall either side not including the corner. I will assume you have the correct settings in Therm with regard to boundary conditions etc.
First, you need to model a section of the wall without the corner in Therm, 1m long is the usual convention, call this wall A . Do not include any of the additional timber associated with the corner. Simulate in Therm to get u-factors, then go to > file >report. Copy and paste all the text from here, (check all U-factors are recorder as total length) into the first column in the spreadsheet, column S. You then repeat this process for the piece of wall the other side of the corner. As this is the same as the 1m section you just competed you can copy the results from the same Therm file into column T, call these u-factors Wall B. Now you construct a new file in Therm that contains the corner junction plus 1m of wall either side, call these u-factors wall C. This is the L2D2, simulate in Therm and copy the results in to column U.
The psi value = wall C – (wall A + wall B). Check through all the boxes in the spread sheet and the calculation should complete for the internal psi value. Adjust the length to calculate the external psi value in the last box.
Hope this makes sense