Quickbird comes in byte and integer format.  Typically, if you are interested in the spectral response of the pixels, you prefer your data in integer format and then convert it to floating point radiance (via the Quickbird radiance tool) or ultimately, reflectance after an atmospheric correction.  If you stretch your data to byte (0-255), then you will diminish its spectral integrity of the data.  If you are only doing visual analysis, then it may not be important but if you are looking at the spectral response of the pixels to quantify something, then it is not advisable to convert your data to byte.  If you do want to do this, you can strech your data into byte using Basic Tools ->Stretch Data. You can perform a Layer Stack with other data via Basic Tools->Layer Stacking.  You will have to decide if you wish to resample the pixel size of ASTER to that of Quickbird, or resample Quickbird to the pixel size of ASTER.  Whichever dataset you import first will be your 'base' image to which the other images are resampled.