Like this:

[environment]

activebpel engine 2.1.1 + tomcat 5.5.27 + win 7

[steps]

1. use assign activity in bpel file
2. deploy process

[error message]

the following error appears in the deployment log (http://127.0.0.1:8080/BpelAdmin/deployment_log_detail.jsp)

Activity annotated with Create Instance must be first basic activity in process

[analysis]

the assign activity is reported as error by org.activebpel.rt.bpel.def.visitors.AeCheckStartActivityVisitor.visit(AeActivityAssignDef aDef) without any checking

[solution]

In org.activebpel.rt.bpel.def.visitors.AeCheckStartActivityVisitor(221), remove:
reportError(aDef);