In Excel, select the object you want to link to and copy it (either by CTRL+c or by "copy" on the ribbon).

In PowerPoint, on the Home panel of the ribbon, click on the word "Paste" (not the clipboard icon, make sure you click below that). Select "Paste Special". Select "Microsoft Excel Worksheet Object" from the list (this phrase might vary if you have a chart or other type of object). To the left, click on the button for "Paste Link".

Click "OK".

Any time the Excel file is updated, your PowerPoint file will reflect the change.

If it doesn't do what you are expecting, post again!