The other answers covered the namespace issue, but I'll add that I found that the context:property-placeholder tag needed to be between "spring:beans" tags. Here's an example that presumes that the properties file sets a property named "jmsBrokerURL":
<mule xmlns="http://www.mulesoft.org/schema/mule/core" version="EE-3.4.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:doc="http://www.mulesoft.org/schema/mule/documentation"
xmlns:spring="http://www.springframework.org/schema/beans"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://www.mulesoft.org/schema/mule/core http://www.mulesoft.org/schema/mule/core/current/mule.xsd
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-current.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd">
<spring:beans>
<context:property-placeholder location="C:/path/to/file/settings.properties" />
<spring:bean name="myConnectionFactory" class="org.apache.activemq.ActiveMQConnectionFactory">
<spring:property name="brokerURL" value="${jmsBrokerURL}" />
</spring:bean>
</spring:beans>
<flow name="MyFlow" doc:name="MyFlow">
<!-- Flow configuration here. -->
</flow>
</mule>
An alternate method of reading properties (and one I prefer) is to use the Spring "util:properties" tag to read properties into a Properties bean which you then refer to using Spring EL. Watch out in this case that you use the Spring EL "#{}" notation instead of "${}" to reference the object and its variables. Here's the above example modified for that technique:
<mule xmlns="http://www.mulesoft.org/schema/mule/core" version="EE-3.4.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:doc="http://www.mulesoft.org/schema/mule/documentation"
xmlns:spring="http://www.springframework.org/schema/beans"
xmlns:util="http://www.springframework.org/schema/util"
xsi:schemaLocation="
http://www.mulesoft.org/schema/mule/core http://www.mulesoft.org/schema/mule/core/current/mule.xsd
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-current.xsd
http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-3.2.xsd">
<spring:beans>
<util:properties id="myConfig" location="C:/path/to/file/settings.properties" />
<spring:bean name="myConnectionFactory" class="org.apache.activemq.ActiveMQConnectionFactory">
<spring:property name="brokerURL" value="#{myConfig.jmsBrokerURL}" /> <!-- Note the pound (hash) symbol. -->
</spring:bean>
</spring:beans>
<flow name="MyFlow" doc:name="MyFlow">
<!-- Flow configuration here. -->
</flow>
</mule>
I like this latter approach mainly because I can deal with multiple properties files and included application context files more easily. The context:property-placeholder tag can be problematic when dealing with either multiple property files or when including an application context file within another.