@Filip:
You didn't post a link to your site, but I guess you're talking about
elody.cz
?
Although it seems that quite a bit of customisation has been made by either the template provider, yourself, or both, the links to previous and next products are in the HTML output.
HikaShop uses by default background images, as defined in the HikaShop frontend_default.css file. Obviously, you're not using this CSS on your site, and you don't have anything set in your template CSS, so you don't see anything.
I don't know your template, and I don't see any typical custom CSS file there, but for starters you could add the following CSS - however you'd do this on your site. Note that this makes use of the default HikaShop images, and it's almost the same as the default HikaShop CSS, just a bit adjusted to what I think makes sense on your site. Of course, you may use other images/icons, and/or alter the values as you prefer.
Once added, you should see left and right arrows below the breadcrumb on product pages.
.hikashop_previous_product{
background-color: transparent !important;
float: left;
width: 6px;
height: 6px;
background: url("http://elody.cz/media/com_hikashop/images/icons/nav_products.png") no-repeat 5px -20px;
padding: 5px;
border: 1px solid #333;
margin: 5px 5px 10px 22px;
}
.hikashop_previous_product:hover{
background: url("http://elody.cz/media/com_hikashop/images/icons/nav_products.png") no-repeat 5px 3px;
}
.hikashop_next_product{
background-color: transparent !important;
float: left;
width: 6px;
height: 6px;
background: url("http://elody.cz/media/com_hikashop/images/icons/nav_products.png") no-repeat -30px -20px;
padding: 5px;
border: 1px solid #333;
margin: 5px 22px 10px 5px;
}
.hikashop_next_product:hover{
background: url("http://elody.cz/media/com_hikashop/images/icons/nav_products.png") no-repeat -30px 3px;
}