The Ecsact Unreal Integration is not available in the unreal marketplace as of yet. Instead we recommend adding a git submodule in your Plugins directory.
# make sure to CD into your project folder that contains your .uproject
cd path/to/your/project/root
# add ecsact integration as a submodule in Plugins/Ecsact
git submodule add https://github.com/ecsact-dev/ecsact_unreal Plugins/Ecsact