Hi David,
after downloading the C&CA vassal module (named "C&CAncientsv3.vmod") remember to delete the previous one (usually named "C&CAncientsv3.mod") otherwise Vassal will continue loading the previous version...
After that unzip the extension file into a folder named "C&CAncientsv3_ext" (it has to be VASSAL_MODULE_NAME with "_ext" suffix).
Now lunching the Vassal module will load all added extensions!