uiimport warning message when specifying a variable to save to?

I am building a GUI, and need to import data from an excel file into a matrix. When I use the command

x = uiimport

the prompts appear to select the file, but the data selection window is different that the one that appears when you just type

uiimport

i.e., it doesn't allow you to specify column vector, matix, cell array. The whole UI is completely simplified. It also comes up with the error message:

Warning: Synchronous evaluation of the uiimport function for spreadsheet files will be
deprecated in a future release. Use the asynchronous uiimport syntax instead. 
> In uiimport at 238

I need to specify the variable so that the code can use it for the remaining calculations, is there a proper syntax for this? I am using version R2012b. Thanks!