Question: You mention downloading AutoCAD 2015. Did you download/install "plain" AutoCAD 2015 or did you download/install AutoCAD Architecture 2015? The former will not have Content Browser, the latter will.
If you have AutoCAD Architecture, but are running it "as AutoCAD", you will need to change your AutoCAD profile. You should have gotten a desktop shortcut that will start AutoCAD Architecture "as AutoCAD Architecture." But if you chose not to add desktop shortcuts when you installed the program, or deleted them afterwards, you can use the OPTIONS command to open the Options dialog. Select the Profiles tab. If you installed AutoCAD Architecture, you should have several profiles that start with "AutoCAD Architecture"; I have AutoCAD Architecture (Global), AutoCAD Architecture (UK), AutoCAD Architecture (US Imperial) and AutoCAD Architecture (US Metric). If you are not located in the United States, you may have different, localized profiles available. Select one of those and click on the Set Current button to set it current. Select OK to ratify the change and return to the drawing. You should see the ribbon change to show the tabs available in that profile.
The Content Browser can be started on the Home ribbone tab, on the Build panel, by choosing the bottom half of the Tools split-button tool and then choosing Content Browser from the drop-down menu. It can also be found on the Insert ribbon tab, on the Content panel. Content Browser is the intial default of the only tool on that panel, if Content Browser is the displayed tool, you can select the top half of the split button. If Building Component or Design Center is the displayed tool (unlike the Home/Build tool, the last-used tool will be displayed here), select the bottom half to deploy the drop-down menu and choose Content Browser.
