Add on not showing up in add blocks list
Have you tried re-installing it?
Does it need additional setup in the dashboard before use?
Is it a dashboard add-on?
It would be easier to help you out if we knew what add-on your are talking about.
1. First, associate an image with a page you want featured. To do this, put the page in Edit Mode and click on Properties. Then click on the Custom Attributes tab. Add the Feature Image attribute. Pick the image you want to use. Hit save, exit Edit Mode and Publish.
2. Next, add a Page List block to the page you want the slider on. Under Numbers and Type of Pages, enter 4 in the Display field (this number must be 4 - inputting a higher number will produce undesirable results). Then, choose the location of where you want the pages displayed from. Finally, check the Truncate Summaries box. The default 128 characters is a good number for that. Click Add to add the Page List block to the page.
3.Now, click on the Page List block that you just added and choose Custom Template from the menu. Then, select either Featured Content Slider Dark or Featured Content Slider Light from the drop down. Hit Update. Exit Edit Mode and Publish your changes.
The add-on doesn't have a block but uses attributes and custom templates for the page list block. That is why you can't add a block after install.
Try to follow the above and I think you will get it to work. If not, then submit a support request in the add-on support section.