You will not be able to install and software or application if you are constantly getting the error Mac OS 10.7 no mountable file systems. The installation file of any file comes in the dmg format which has to mounted to any Mac drive and then it will get installed in the Mac system. But if the file can not be mounted in the drive and showing mac os 10.7 no mountable file systems error then you can not use the software. You must first check if the dmg file is corrupted or not, you can get the fresh copy of the file from the distributer. But if the file is okay then ther issue could be with the Mac system. You can check the problem with the help of inbuilt disl utility which will try to fix it. Sometimes it may fail in that case you have to format that drive by using disk utility only. After formatting mac os 10.7 no mountable file systems error will be resolved but the stored data will be erased. You can get back the data by using the time machine or Mac data recovery software which ensures complete and easy recovery.