When you refer to another survey in a relevance or choice lookup you need to use its identifier. Something like "s12_3455". Unlike the survey's name this will uniquely identify the survey that you are referring to. The following tools can be used to help find and interpret this identifier.
If you are editing a survey that refers to another survey's identifier then you can select that identifier and click on Tools then Survey Lookup to find the project and name of the referenced survey.
If you want to insert a survey ident into a pulldata function or similar, then you can click on Tools then Get Survey Ident. In the dialog use the tab keys to navigate down to the survey you want and then press the return key. The survey identifier will be copied to the clipboard and you can then paste it into the editor.