I assume you downloaded the version of Data Modeler that does not include a JRE?
The Java JRE should normally contain a copy of msvcr71.dll in its folder jre\bin.
If it's missing, you will probably find another copy in its folder jre\bin\new_plugin. You could try copying this to its jre\bin folder.
If you've still got a problem, you could download the version of Data Modeler that includes a JRE.
Run regedit as the Administrator
Expand HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths
Add a new Key to App Paths called sqldeveloper.exe
Set the (Default) value to the full path to the executable (including the executable name).
Create a new String value for sqldeveloper.exe called Path and set its value to the jre / bin folder in the SQLDeveloper files.