Is this the complete XML?
It’s missing a close element for <SourcedFixletAction> and <BES>. Try with a file with contents:
<?xml version="1.0" encoding="UTF-8"?>
<BES xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="BES.xsd">
<SourcedFixletAction>
<SourceFixlet>
<Sitename>master</Sitename>
<FixletID>79706</FixletID>
<Action>Action1</Action>
</SourceFixlet>
<Target>
<ComputerName>testcomputer.domain</ComputerName>
</Target>
<Parameter Name="username">tester</Parameter>
<SecureParameter Name="password"><![CDATA[test13_password]]></SecureParameter>
</SourcedFixletAction>
</BES>