Is the message in the app store you are seeing the following:
The ... module cannot be downloaded because it was created with version 5.18.0 of the modeler...Please open with any version of the modeler 6.0-6.10.9
If so, the issue isn't that it won't work with version 7, but rather it needs to be upgraded to version 6 first. This will happen automatically if you add the module to a version 6 project. To get it to work, try the following:
1) Create a Mendix project in version 6
2) download the app store module for forgot password into your project.
3) export it from Mx6 and import it into your Mendix 7.7 project.
This will let the platform do all the conversion for you. If instead the issue is that you already have the module, but the widget isn't able to be put on a page, then you will have to write a microflow that does some of what the module does. Most of the module should work just fine in Mendix 7.
Hope this helps,