If you place your BPMN 2.0 process definition XML somewhere in the Alfresco classpath, you can use Alfresco's workflow console to deploy the definition.
For example, I always place my workflows under WEB-INF/classes/alfresco/extension/workflows/someFolder where someFolder is a unique folder for each process definition I am using.
The workflow console is in http://localhost:8080/alfresco/faces/jsp/admin/workflow-console.jsp. Assuming you are using 3.4.e, which is a preview release showing Activiti integration, you can deploy a process through the workflow console with this command:
deploy activiti /alfresco/extension/workflows/activiti/activitiHelloWorld.activiti
You can see other helpful workflow console commands by typing help.
Alternatively, as Gagravarr suggests, you can use Spring to deploy your workflow when Alfresco starts up. The Spring config file must have a name ending with "-context.xml". I usually place mine in WEB-INF/classes/alfresco/extension.
<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE beans PUBLIC '-//SPRING//DTD BEAN//EN' 'http://www.springframework.org/dtd/spring-beans.dtd'>
<beans>
<bean id="someco.workflowBootstrap" parent="workflowDeployer">
<property name="workflowDefinitions">
<list>
<props>
<prop key="engineId">activiti</prop>
<prop key="location">alfresco/extension/workflows/activiti/activitiHelloWorld.bpmn20.xml</prop>
<prop key="mimetype">text/xml</prop>
<prop key="redeploy">false</prop>
</props>
</list>
</property>
<property name="models">
<list>
<value>alfresco/extension/model/scWorkflowModel.xml</value>
</list>
</property>
<property name="labels">
<list>
<value>alfresco.extension.messages.scWorkflow</value>
</list>
</property>
</bean>
</beans>
If you'd like working examples of some simple workflows, with the same workflows implemented for both jBPM and Activiti for easy comparison, take a look at this blog post: http://ecmarchitect.com/archives/2011/04/27/1357
Jeff