Follow the steps in the Esri KB article:
http://resources.arcgis.com/content/kbase?fa=articleShow&d=35515
WebOffice 10 R3 is certified for ISAPI Redirector 1.2.37.
Specific steps to be applied when working with IIS 7:
R. In IIS Manager, on the left panel of the window, navigate to "Default Website" and click it, then double-click "Authentication" and click "Anonymous Authentication". Verify that it is "enabled".
S. Navigate back to "Default Website", expand it and select "Jakarta". Double-click "Authentication" from the middle panel. Click "Anonymous Authentication". Verify that it is "enabled".
T. Click "Jakarta" again. Double-click "Handler Mappings". If ISAPI.dll is disabled, select "Edit Feature permissions" from the right-hand side and enable "Execute".
Important note: In case the ISAPI Redirector does not work properly, then you may use the Tomcat port directly (this way you do not need IIS at all). If e.g. you did set the standard Tomcat port throughout installation (8080) then starting a WebOffice 10 R3 project would look like this:
http://mymachine:8080/WebOffice/synserver?project=WebOffice_SampleProject