Trying to Open .sln,.SLN File Greyed Out - Can't Open

When I open Unity Version 2017.3.0f3 Personal, and then click open, navigate to my .sln file (this was downloaded as part of a class I'm taking on Coursera, but I've never had an issue before) it's greyed out. In fact, all of the .sln files I've opened in the past are now greyed out. The only way I can successfully open something is by selecting one of the recent options it provides. I'm running High Sierra (yes it's up to date) on a MacBook Pro. Please help!,When I try to open a .sln file in Unity that I downloaded for a class I'm taking from Coursera, it's greyed out. I'm on a Mac running High Sierra and Unity Version 2017.3.0f3 Personal. Please help, I have no idea what's wrong and really need to get my assignments done!