Remember that Lertap5 is an "app" which runs with Microsoft Excel. Your computer must have a copy of Excel in order to be able to run Lertap5. Excel is part of the suite of programs which Microsoft refers to as "Office". Other apps in the Office suite include Word, Access, and PowerPoint.
There is a version of Lertap5 for Windows users, and also a version for Mac users. The two versions are very similar but not quite identical.
Note this potential problem for Windows users: Microsoft significantly enhanced security scans for Excel workbooks in late 2018. These can affect the installation of Lertap5. After downloading and installing Lertap5, if Excel then takes a long time to open Lertap5, more than 4 or 5 minutes, please refer to this topic for a fix. (Generally Lertap5 will open in less than 30 seconds on a Windows computer.)
Get Lertap5 for Windows by downloading the installer file using this link. The file has a name similar to "SetupLertap51094.exe" -- the code number (in this case) stands for Lertap5, version 10, edition 9, revision 4. Open and run the file after the download has completed; if needed, refer to this topic for installation assistance.
To get a copy of Lertap5 for a Macintosh computer, read this document and follow its instructions -- note that this version requires the latest version of Excel for the Mac (Excel 2016/2019, also known as Excel 365 in some cases).
Excel "365" refers to a version of Excel, usually the very latest version, covered by an annual subscription.
A document covering the on-going development of Lertap5, with version numbering, is available here.
The Macintosh version will usually also work with the Windows version of Excel 2016/2019, but it is somewhat slower when running under Windows, and lacks a bit of functionality in places.