Steaks are among the most prime cuts of meat because they are taken from the most tender, juicy parts of the animal. These tender cuts benefit from searing over high heat and fast cooking, where the tenderness is least affected.
Slow cookers are usually used upon tough cuts that are not suitable for quick searing like steaks, but rather rely on long cooking to break them down into tenderness. These cuts are usually cheaper because they are less desirable, tougher, require long cooking times like braising or pot roasting.
If you have only a microwave and slow cooker then you should focus on recipes that benefit them. In this case instead of buying steaks you should buy the cheaper, tougher, less expensive cuts and then use your slow cooker to advantage and cook them for long times (several hours).
So in a word about cooking steaks in a slow cooker: don't.
Waste of time, waste of money.