I am in QGIS trying to combine a Zip code and City (separate columns) together and my methods are not working. Here is a screenshot of the issue.

I am in QGIS trying to combine a Zip code and City (separate columns) together and my methods are not working. Here is a screenshot of the issue.

String concatenation using the + operator has only been added in 2.6. For all older versions you have to use || to concatenate strings.
See also Is there a way to concatenate fields in field calculator of QGIS?