When you are ready to test, go to File > Upload to FTP host
Enter your FTP address, login and Password, then click next.
Now on the first field (site URL) enter the address of your website, where you will upload your website eventually after the test, then type the name of your test folder followed by the ( / ) sign.
For Example, if your website URL is www.yoursite.com and you want to test it in a folder that is called TestFolder then enter www.yoursite.com/TestFolder
In the next field (Folder on Server), type ONLY the name of your test folder followed by the ( / ) sign.
If this is the first time you are uploading to this folder, Muse will ask you that if you want to create this folder. You can click OK and the folder will be created on your FTP.
When you are done testing, remove the /TestFolder (in step 3), upload again, and you will have your website on your main url.