You may need to unzip the file, especially if you downloaded a shapefile (it's almost guaranteed to be in a zipped folder). Although in Windows Explorer you can double-click on a zipped file and see what’s inside, ArcGIS can’t read that data until it’s unzipped.
To unzip a folder, right click on the zipped folder and select Extract All… from the resulting menu, then choose where you want to extract it. Or use your favourite unzipping software (like WinZip or 7zip).
If the file isn't zipped or otherwise compressed (.zip, .tar, .7z, etc.), try using the Add Data button instead of dragging and dropping.
Best practice: always unzip files (.zip, .tar or .7z) and use the Add Data button in ArcMap to add files to your project.
ArcGIS can be finicky about filenames and folder names and may give you errors if you have a space, dash, or period in the filename. Save yourself the trouble and always use underscores or CamelCase when naming your project folders and files.
Have a look at our help guide on GIS file formats and compatible softwares as you might find the filetype there (e.g.: .adf, .tab, .dem, a folder with a whole bunch of files like a0000001.gdbindexes) as well as how to view it in ArcMap. If it's not there or on this list of compatible ArcMap data formats, and you don't have any luck searching Google for that filetype/file extension and ArcMap, let us know and we'll help.
The Red Exclamation Mark of Doom next to a layer (or all the layers, yikes!) in your ArcGIS map document (.mxd) can be a stressful thing but it truly doesn’t need to be: all it means is that your data is no longer correctly linked to your MXD and it is easy to reconnect to the datasets:
Right click on the layer with the Red Exclamation Mark of Doom and select Properties (near bottom of menu).
Click the Source tab (second from left), then the Select Data Source button to browse to the correct file (shapefile, geodatabase, DEM, etc.).
Once you find the file, click OK.
To avoid this issue in the first place, best practice is to create a project folder and subfolders for data. Maintaining that structure and clicking File—Map Document Properties in ArcGIS, then clicking the ‘Store relative pathnames to data sources’ box will help.
Remember: when you copy and/or move your project, you need the MXD and the data! Data and MXD need to be cut/copied/transferred in tandem. This means all of those shapefile components, too.
The default colour that ArcMap uses for selected features is a bright blue and the features will stay selected until you unselect them, no matter what your symbology choice is. To solve this, click Selection in the menu bar then Clear Selected Features.
The error probably looks like this:
If you have an Esri license from Carleton, all the extensions should be licensed but you need to enable them. Go to Customize > Extensions... in the menu bar and select the extension(s) you want to activate (the error usually tells you which extension is required). Generally speaking, we recommend having the 3D Analyst, Data Interoperability, and Spatial Analyst extensions enabled.