First of all, a lookup field is probably not the correct control in your use case.
What do you want to achieve? Display the location of the connected user?
If yes, I would use ca_reportQuery in a function called onLoad of your form, then use the return to populate a textbox.
Louis_van_Amelsfort answers will help you a lot.
Finally, to answer your question, in Report /plug-in variables of a control that support it, set : $({'STRING':_.user.id}). (This is not feasible in a lookup btw, so this is why i said that you do not use the correct control)
To complete the answer i will use Louis_van_Amelsfort code snippet and modify it a lil'bit to suit your needs :
{
onLoad : function() {
ca_fd.js.setInfo();
},
setInfo : function() {
ca_reportQuery('yourReportObjectID', {'STRING':_.user.id}, ca_fd.js.onSuccess, ca_fd.js.onFailure);
},
onSuccess : function(result) {
if (result.length == 1) {
ca_fdSetTextFieldValue(ca_fd.formId, 'email_addr', result[0].email_address);
}
else {
ca_fdSetTextFieldValue(ca_fd.formId, 'email_addr', 'Cannot find user details');
}
},
onFailure : function(){
ca_fdSetTextFieldValue(ca_fd.formId, 'email_address', 'Failed');
},
}
Simply add : ca_fd.js.onLoad(); into the onLoad attribute of your form.
Hope this helps.