Are you trying to set up a static study or non-linear? The static study will not give you the results you are looking for and I don't think non-linear handles time-step. What are you simulating (stress? displacement? fatigue? volume change? fluid?) and we will be better suited to help.
Hi Antony: if you set up a time history (dynamic) analysis, the loads dialog has an input option to specify the load as a function of time. You could make an Excel spreadsheet with values and import it, or just type-in values.