Extract (I used Universal Extractor) the downloaded .exe file (DoubleCADUniversal_v3_1.exe in my case). You will end up with two folders and a file. Keep the "XT" folder and delete the rest. Inside the "XT" folder delete everything except the .cab and .msi file. Use the command string below for a silent install. Make sure you modify the .msi file name, PIDKEY, USERNAME AND COMPANYNAME to match yours

When you get the download from IMSI it will have four sets of numbers. You only need the serial number for the DoubleCAD XT.

msiexec /i "DoubleCAD XT 3.msi" /qn PIDKEY=xxxx-xxxx-xxxx-xxxx USERNAME="user" COMPANYNAME="company"