After a new install it can happen that the default page doesn't have a login block. It just shows the titles "Login" and "Main menu", but not the active blocks.

Since you can't login as administrator (allthough you have just registered an administrator name and password), you need to start debug mode by accessing the database directly (using phpMyAdmin or similar).

See here how to do that: Enter debug mode manually

Try again and follow the resulting error message to find a solution.

One possible cause is the collation of the Xoops database you have created. If you are using utf8 unicode, try to switch to a Latin character set, drop all tables, make a fresh copy of the Xoops files and re-install from scratch.

There are other causes, such as MySQL not having write access to a tmp folder. See this post for further guidance:

Login block missing