Use either the normalization process suggested by @SBeumer, or if you need to compute other measures (eg. ratio of two frequency bands), you can use the generic process "Pre-process > Run Matlab command"
how can I represent it in milliseconds and make the spacing smaller (such as, -500ms, -250ms, 0ms, 250ms, 500ms)
Indeed, you can edit the properties of the axes object, either interactively (Figure > Matlab controls) or from the command line. Look for help in the Matlab documentation.